[camera_avfoundation] Fix crash when streaming while recording

[RobertOdrowaz]

In captureOutput, we receive both pixel and audio buffers (audio only during recording with audio enabled, when streaming alone is enabled, everything works correctly). Currently, the streaming part of this method doesn't handle non-pixel buffers properly, causing a crash because of force unwrapping of nil.

This PR changes the logic to ignore non-pixel buffers for purposes of streaming.

Resolves flutter/flutter#172894.

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[gemini-code-assist]

Code Review

This pull request fixes a crash that occurs when streaming is enabled while recording video. The crash was caused by force-unwrapping a nil value when trying to get a pixel buffer from an audio sample buffer. The fix correctly handles this by using optional binding to check if a pixel buffer exists before processing it. The change is accompanied by a new unit test that verifies audio buffers are ignored during image streaming. My feedback includes a suggestion to also handle a potential nil value for planeAddress to prevent another potential crash.

[gemini-code-assist]

There's a potential crash here if planeAddress is nil. The ! force-unwraps it when creating Data. It's safer to use optional binding (if let or guard let) to ensure planeAddress is not nil before using it.

            if let planeAddress = planeAddress {
              let length = bytesPerRow * height
              let bytes = Data(bytes: planeAddress, count: length)

              let planeBuffer: [String: Any] = [
                "bytesPerRow": bytesPerRow,
                "width": width,
                "height": height,
                "bytes": FlutterStandardTypedData(bytes: bytes),
              ]
              planes.append(planeBuffer)
            }

[RobertOdrowaz]

The only case in which we should get nil planeAddress is if we use incorrect function CVPixelBufferGetBaseAddress or CVPixelBufferGetBaseAddressOfPlane for a given buffer. We are checking the whether buffer is planar so this should never happen. I don't think failing silently is necessarily better in this case

[hellohuanlin]

can you use guard here?

[RobertOdrowaz]

We couldn't use guard there because this method also handled recording later, so an early return would break audio recording. I've extracted the whole streaming part to handleSampleBufferStreaming and replaced all nested ifs with guards. I left the pixel buffer guard separate so that I can comment on it