Read correctly empty configuration file

Summary:
When a configuration file exists but is empty, YAML parser returns None.

This change returns an empty dict to represent the empty configuration.

This fixes the following issue for empty config file:

File "/home/dereckson/dev/python/general/lib64/python3.13/site-packages/mergedictionaries/app/app.py", line 118, in <lambda>
  self.context["config"].get("git", []), self.context["git"]
  ^^^^^^^^^^^^^^^^^^^^^^^^^^

AttributeError: 'NoneType' object has no attribute 'get'

Test Plan:

• create an empty configuration file
• merge-dictionaries --extract

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3725