Pass empty string to body_path if changes-file is empty

[yksen]

Hi, the problem is related to #5. Basically when inputs.changes-file variable is empty then body_path for softprops/action-gh-release is set to inputs.working-directory which results in an Error: EISDIR: illegal operation on a directory, read trying to read a directory.

Failing job example

...
changes-file = 
...
Run softprops/action-gh-release@v2
  with:
    body_path: ./
...
⚠️ Unexpected error fetching GitHub release for tag refs/tags/v0.2.1: Error: EISDIR: illegal operation on a directory, read
Error: EISDIR: illegal operation on a directory, read

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Passing an empty string to body_path solves the issue.