fix(enc-mediafoundation): Handle null samples from ProcessOutput gracefully

[seer-by-sentry]

Fixes CAP-DESKTOP-K6. The issue was that: H264 Media Foundation Transform returned success from ProcessOutput but provided a null sample pointer, causing unwrap() panic.

• ProcessOutput may succeed but not populate pSample in some edge cases.
• Handle this gracefully by skipping the frame instead of panicking.

This fix was generated by Seer in Sentry, triggered by Richie. 👁️ Run ID: 1916819

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