fix(magic): handle null encoding to prevent decode TypeError

[KevinLiebergen]

This PR fixes a TypeError that occurs when PolyFile processes certain binary files (specifically observed with PNG, JPG, and GIF images).

The Problem: In polyfile/magic.py, the PlainTextTest.test method uses chardet.UniversalDetector to guess the encoding of a file. In some cases, the detector returns a confidence score higher than the minimum threshold but sets the encoding result to None.

When this happens, the following line triggers a crash because decode() expects a string:

value = data[absolute_offset:].decode(encoding)  # encoding is None

The Fix: Added a check to ensure detector.result["encoding"] is not None before attempting to use it for decoding. If it is None, the test correctly falls back to a FailedTest, allowing PolyFile to continue its analysis instead of exiting with a traceback.

Verification: Verified by running polyfile against several image files that previously triggered the crash. The analysis now completes successfully for all files.

[CLAassistant]

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