Fix empty plots when first track is not a video track #57

Job	Run time
build-linux (gcc-9)	6m 55s
build-macos-brew (clang-9)	2m 57s
build-linux-Qt6 (gcc-9)	8m 11s
build-macos-brew-Qt6 (clang-9)	3m 32s
build-macos-no-brew (clang-9)	4m 42s
build-macos-no-brew-Qt6 (clang-9)	5m 47s
build-windows-mingw (5.15.*)	9m 55s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	11m 28s
build-windows-vs (5.15.*)	8m 50s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	13m 29s
	1h 15m 46s

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