Return null for unavailable compressed probe images

[gregbenz]

Return null for unavailable compressed probe images

Summary

• return nullptr from the compressed base and gain-map getters when probing did not provide a payload
• preserve existing JPEG behavior when compressed item data is available
• add public-API coverage for JPEG, HEIF, and AVIF

Why

uhdr_dec_get_base_image() and uhdr_dec_get_gainmap_image() document that they return nullptr when compressed image data is unavailable. HEIF/AVIF probing currently does not populate those compressed payloads, but the getters return a non-null descriptor with data == nullptr and data_sz == 0.

Callers following the documented pointer contract can therefore mistake an unavailable payload for valid compressed data and pass a null data pointer downstream.

This change returns nullptr unless a non-empty compressed payload is present. JPEG probing behavior is unchanged.

Testing

• verified JPEG probing still returns non-empty compressed base and gain-map payloads
• verified HEIF and AVIF probes return nullptr for unavailable compressed payloads
• complete unit suite passes

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