Incorrect data source folders when config has no path

[modwyer42]

When using custom paths for the data files and then running mkdocs build with no path to the config file, the data source folders list is created with an incorrect file structure.

Example:
mkdocs.yaml file include the block:

  - markdownextradata:
      data: docs/source_data

and the build is run from the base folder without a path for config:

mkdocs build -f mkdocs.yml

The base_path in the plugin will be set to '' and the f-string in the list comprehension will build the list with paths like /docs/source_data rather than either docs/source_data or ./docs/source_data

Given the lack of a path to config in the CLI is equivalent to ./, you could potentially set the base_path to the config dirname or '.' if it's an empty string.