text: Title: Improve error message when pipeline.yaml has empty tasks
  Body: ```yaml
  # pipeline.yaml
  
  tasks:
    # Add tasks here...
  ```
  
  Error is pretty confusing:
  
  ```pytb
  Traceback (most recent call last):
    File "/Users/Edu/miniconda3/envs/soopervisor/bin/soopervisor", line 33, in <module>
      sys.exit(load_entry_point('soopervisor', 'console_scripts', 'soopervisor')())
    File "/Users/Edu/miniconda3/envs/soopervisor/lib/python3.8/site-packages/click/core.py", line 829, in __call__
      return self.main(*args, **kwargs)
    File "/Users/Edu/miniconda3/envs/soopervisor/lib/python3.8/site-packages/click/core.py", line 782, in main
      rv = self.invoke(ctx)
    File "/Users/Edu/miniconda3/envs/soopervisor/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
      return _process_result(sub_ctx.command.invoke(sub_ctx))
    File "/Users/Edu/miniconda3/envs/soopervisor/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
      return ctx.invoke(self.callback, **ctx.params)
    File "/Users/Edu/miniconda3/envs/soopervisor/lib/python3.8/site-packages/click/core.py", line 610, in invoke
      return callback(*args, **kwargs)
    File "/Users/Edu/dev/soopervisor/src/soopervisor/cli.py", line 64, in add
      Exporter.new('soopervisor.yaml', env_name=env_name, preset=preset).add()
    File "/Users/Edu/dev/soopervisor/src/soopervisor/abc.py", line 282, in new
      dag, spec = load_dag_and_spec(env_name)
    File "/Users/Edu/dev/soopervisor/src/soopervisor/commons/dag.py", line 175, in load_dag_and_spec
      spec, _ = commons.find_spec(cmdr=cmdr, name=env_name)
    File "/Users/Edu/dev/soopervisor/src/soopervisor/commons/dag.py", line 59, in find_spec
      spec, relative_path = DAGSpec._find_relative()
    File "/Users/Edu/dev/ploomber/src/ploomber/spec/dagspec.py", line 499, in _find_relative
      return cls(relative_path), relative_path
    File "/Users/Edu/dev/ploomber/src/ploomber/spec/dagspec.py", line 195, in __init__
      self._init(data=data,
    File "/Users/Edu/dev/ploomber/src/ploomber/spec/dagspec.py", line 340, in _init
      self.data['tasks'] = [
  TypeError: 'NoneType' object is not iterable
  ```