[PYTHON] fix: load_config now returns empty dict instead of None when empty file

[kacpermuda]

Problem

Thanks to @rahul-madaan that noticed it. When an empty openlineage.yml is found, python client crashes during creation as the self.config is non-iterable (None).

Solution

Make sure the load_config() always returns a dict.

One-line summary:

utils.load_config() now returns empty dict instead of None when empty file to prevent OpenLineageClient crash

Checklist

• You've signed-off your work
• Your pull request title follows our guidelines
• Your changes are accompanied by tests (if relevant)
• Your change contains a small diff and is self-contained
• You've updated any relevant documentation (if relevant)
• Your comment includes a one-liner for the changelog about the specific purpose of the change (if necessary)
• You've versioned the core OpenLineage model or facets according to SchemaVer (if relevant)
• You've added a header to source files (if relevant)

---

SPDX-License-Identifier: Apache-2.0
Copyright 2018-2023 contributors to the OpenLineage project

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.84%. Comparing base (c9b06a4) to head (3308a69).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2596   +/-   ##
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  Coverage   84.84%   84.84%           
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  Files          59       59           
  Lines        3398     3398           
=======================================
  Hits         2883     2883           
  Misses        515      515           

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