fix: default to YUV_400 format for .raw files

[for13to1]

Summary of Changes

As the author who originally contributed the RAW file support feature, this PR fixes a format fallback inconsistency for .raw (RAW Bayer / CFA patterns) files and resolves a potential empty optional dereference crash.

Background & Rationale

YUView uses YUV_400 (luma-only) internally to read and display single-channel raw sensor files (such as Bayer raw, defined in RAW_BAYER_EXTENSIONS).

Previously, if a .raw file had no resolution in its filename, it would bypass format guessing and fallback to the default YUV_420 format from videoHandlerYUV's constructor. This required users to manually select 400 (YUV 4:0:0) in the properties GUI.

This PR sets the default format to YUV_400 for files matching RAW_BAYER_EXTENSIONS right at construction, so they default to the correct single-channel interpretation.

Detailed Changes

1. Fallback Consistency: Set the default format for RAW_BAYER_EXTENSIONS to YUV_400 in the playlistItemRawFile constructor.
2. Robustness: Added a safety check in checkSpecificFileExtensions (PixelFormatYUVGuess.cpp) to prevent undefined behavior when frameSize is empty.

Verification

• Built successfully and verified that opened .raw files now default to YUV 4:0:0 in the properties GUI.
• Passed all 1860 unit tests.

[for13to1]

#663