text: Title: validate import_tasks_from
  Body: when loading the `import_tasks_from: file.yaml`, we are not validating its contents, if the file is empty, `yaml.safe_load` will return `None`, which will throw a cryptic error message:
  
  https://github.com/ploomber/ploomber/blob/beb625cc977bcd34481608a91daddc5493e0983c/src/ploomber/spec/dagspec.py#L326
  
  Fix:
  * If `yaml.safe_load` returns None, replace it with an empty list
  * If returns something other than a list, raise an error saying we were expecting a list
  