gh-152959: Fix IndexError when <key> appears outside <dict> in plistlib

[Wojusensei]

Closes #152959

when parsing an XML plist file, if a <key> element appears outside of a <dict> container, the parser currently raises an IndexError because the stack is empty. This error is confusing and doesn't help users understand the real issue

this pr adds a check at the start of end_key() to verify that the stack is not empty. if it is empty, we raise InvalidFileException with a clear message that includes the line number, so users immediately know the plist file is malformed

4 example the following invalid plist now produces a meaningful error instead of an IndexError:

foo bar

• Issue: plistlib: a <key> outside a <dict> raises IndexError instead of a proper error #152959

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[python-cla-bot]

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[Wojusensei]

can someone plz add the skip news label?
this is a small bug fix that doesnt need a separate NEWS entry :)

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