Check for empty pillar cache disk file and not call msgpack to load it, with test

[dmurphy18]

What does this PR do?

Check file is not empty before attempting to read pillar disk cache file

What issues does this PR fix or reference?

Fixes: #63729

Previous Behavior

Would have errors in the log file

File "msgpack/_unpacker.pyx", line 200, in msgpack._unpacker.unpackb
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xfe in position 0: invalid start byte

New Behavior

No errors since don't ask msgpack to load an empty file

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

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[Ch3LL]

Why are there empty files in the first place? Would a better fix be to ensure we aren't adding empty files into the cache?

[dmurphy18]

@Ch3LL This is the pillar cache on disk that is causing the issue to occur, the file has been created but has not been filled with contents, hence it is empty and when the code requests the msgpack to load it, the error is thrown by msgpack since it doesn't handle a file with no contents.
Easiest solution is to not ask msgpack to load an empty file.