Camera: Align end-of-stream behavior across platforms

Capturing sessions using camera should mark the end of the capture-session with a default-constructed QVideoFrame. Currently this behavior is inconsistent across platforms.

Each platform should be modified to emit this null-frame after the final frame has been sent. An easy way to confirm whether it is working correctly is to test using the endOfStreamPolicy of QML VideoOutput.

Additionally, this null-frame should preferably be emitted from the same thread as the one that emits the real frames, as to avoid race conditions or frames appearing out of order.

This ticket can be closed once this behavior has been aligned on the following platforms:
[ ] - Android
[ ] - iOS / macOS
[ ] - Linux
[ ] - Windows
[ ] - Embedded