text: Title: DVC log_plot error is confusing, if the `template` attribute is set without a value.
  Body: # Bug Report
  
  ## Description
  
  DVC `log_plot` error is confusing, if the `template` attribute is set without a value.
  
  ### Reproduce
  1. Create a DVC repository
  2. Create a sample experiment that uses the `log_plot` function to log a few metrics (Note: the `template` option is empty)
  ```python
  import pandas as pd
  from dvclive import Live
  from sklearn.datasets import load_iris
  
  iris = load_iris()
  datapoints = pd.DataFrame(data=iris.data, columns=iris.feature_names)
  
  with Live() as live:
      live.log_plot(
          "sepal",
          datapoints,
          x="sepal length (cm)",
          y="sepal width (cm)",
          template="",
          title="Sepal width vs Sepal length")
  ```
  3. Run the experiment
  4. Error will be something similar to the one below
  ```bash
  vscode ➜ /workspaces/processor/.dvc/tmp (feature/strategy3) $ dvc plots diff -v
  2024-07-14 09:57:01,538 DEBUG: v3.51.2 (pip), CPython 3.10.13 on Linux-6.6.31-linuxkit-x86_64-with-glibc2.31
  2024-07-14 09:57:01,538 DEBUG: command: /home/vscode/.local/bin/dvc plots diff -v
  2024-07-14 09:57:03,690 ERROR: unexpected error - [Errno 21] Is a directory: '/workspaces/processor/.dvc/tmp'                                                                                                                              
  Traceback (most recent call last):
    File "/home/vscode/.local/lib/python3.10/site-packages/dvc/cli/__init__.py", line 211, in main
      ret = cmd.do_run()
    File "/home/vscode/.local/lib/python3.10/site-packages/dvc/cli/command.py", line 27, in do_run
      return self.run()
    File "/home/vscode/.local/lib/python3.10/site-packages/dvc/commands/plots.py", line 107, in run
      renderers_with_errors = match_defs_renderers(
    File "/home/vscode/.local/lib/python3.10/site-packages/dvc/render/match.py", line 131, in match_defs_renderers
      renderer = renderer_cls(plot_datapoints, renderer_id, **first_props)
    File "/home/vscode/.local/lib/python3.10/site-packages/dvc_render/vega.py", line 87, in __init__
      self.template = get_template(
    File "/home/vscode/.local/lib/python3.10/site-packages/dvc_render/vega_templates.py", line 724, in get_template
      with _open(template_path, encoding="utf-8") as f:
  IsADirectoryError: [Errno 21] Is a directory: '/workspaces/processor/.dvc/tmp'
  
  2024-07-14 09:57:03,752 DEBUG: link type reflink is not available ([Errno 13] Permission denied: '/workspaces/.VcJKvhyzGauYU48vSH9qxQ.tmp')
  2024-07-14 09:57:03,752 DEBUG: Removing '/workspaces/.VcJKvhyzGauYU48vSH9qxQ.tmp'
  2024-07-14 09:57:03,753 DEBUG: link type hardlink is not available ([Errno 95] no more link types left to try out)
  2024-07-14 09:57:03,753 DEBUG: Removing '/workspaces/.VcJKvhyzGauYU48vSH9qxQ.tmp'
  2024-07-14 09:57:03,753 DEBUG: link type symlink is not available ([Errno 13] Permission denied: '/workspaces/processor/.dvc/cache/files/md5/.WaF1GsOTeDBjuMNIcuPZpw.tmp' -> '/workspaces/.VcJKvhyzGauYU48vSH9qxQ.tmp')
  2024-07-14 09:57:03,753 DEBUG: Removing '/workspaces/.VcJKvhyzGauYU48vSH9qxQ.tmp'
  2024-07-14 09:57:03,754 DEBUG: Removing '/workspaces/processor/.dvc/cache/files/md5/.WaF1GsOTeDBjuMNIcuPZpw.tmp'
  2024-07-14 09:57:03,761 DEBUG: Version info for developers:
  DVC version: 3.51.2 (pip)
  -------------------------
  Platform: Python 3.10.13 on Linux-6.6.31-linuxkit-x86_64-with-glibc2.31
  Subprojects:
          dvc_data = 3.15.1
          dvc_objects = 5.1.0
          dvc_render = 1.0.2
          dvc_task = 0.4.0
          scmrepo = 3.3.6
  Supports:
          http (aiohttp = 3.9.5, aiohttp-retry = 2.8.3),
          https (aiohttp = 3.9.5, aiohttp-retry = 2.8.3),
          s3 (s3fs = 2024.6.1, boto3 = 1.34.131)
  Config:
          Global: /home/vscode/.config/dvc
          System: /etc/xdg/dvc
  Cache types: 
  Cache directory: fakeowner on /run/host_mark/Users
  Caches: local
  Remotes: s3
  Workspace directory: fakeowner on /run/host_mark/Users
  Repo: dvc, git
  Repo.site_cache_dir: /var/tmp/dvc/repo/3e826ad9ce4b0c5c7907b07975f63692
  ```
  
  ### Expected
  Error should point out that a `template` needs to be specified if the attribute is specified.
  
  ### Environment information
  **Output of `dvc doctor`:**
  
  ```console
  DVC version: 3.51.2 (pip)
  -------------------------
  Platform: Python 3.10.13 on Linux-6.6.31-linuxkit-x86_64-with-glibc2.31
  Subprojects:
          dvc_data = 3.15.1
          dvc_objects = 5.1.0
          dvc_render = 1.0.2
          dvc_task = 0.4.0
          scmrepo = 3.3.6
  Supports:
          http (aiohttp = 3.9.5, aiohttp-retry = 2.8.3),
          https (aiohttp = 3.9.5, aiohttp-retry = 2.8.3),
          s3 (s3fs = 2024.6.1, boto3 = 1.34.131)
  Config:
          Global: /home/vscode/.config/dvc
          System: /etc/xdg/dvc
  Cache types: 
  Cache directory: fakeowner on /run/host_mark/Users
  Caches: local
  Remotes: s3
  Workspace directory: fakeowner on /run/host_mark/Users
  Repo: dvc, git
  Repo.site_cache_dir: /var/tmp/dvc/repo/3e826ad9ce4b0c5c7907b07975f63692
  ```
  
  **Additional Information (if any):**
  
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