Add YouTubeResponseIsEmpty exception to handle empty YouTube responses

[etienneschalk]

Title
➔ Add YouTubeResponseIsEmpty exception to handle empty YouTube responses

Description
This PR introduces a new YouTubeResponseIsEmpty exception, derived from CouldNotRetrieveTranscript.
It is raised when a request to YouTube succeeds but the response content is empty, making transcript parsing impossible.

Main changes:

• Added the YouTubeResponseIsEmpty class in _errors.py.
• Updated _transcripts.py to check if the response content is empty before attempting to parse it.
• Improved error handling for cases where YouTube returns an empty response without an HTTP error.

This enhancement provides clearer diagnostics for this specific failure scenario and suggests retrying later, as indicated in the exception message.

[coveralls]

Pull Request Test Coverage Report for Build 14679514702

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 99.821%

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Files with Coverage Reduction	New Missed Lines	%
_transcripts.py	1	99.55%

Totals
Change from base Build 14612450470:	-0.2%
Covered Lines:	559
Relevant Lines:	560

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💛 - Coveralls