video-color: Deal with NULL colorimetry while converting from string

This came up in the case where v4l2 sets caps with colorimetry=NULL, and
then tries to parse back the colorimetry, causing a crash in
gst_video_get_colorimetry() because of g_str_equal(). We fix this by
making sure the only caller of the function never calls it with a null
colorimetry string.
19 jobs for colorimetry-fixes in 59 minutes and 19 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
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gst indent

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manifest

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  Build
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build fedora x86_64

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build nodebug fedora x86_64

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check fedora

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integration testsuites fedora

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  Full Builds
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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  Full Tests
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valgrind bad

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valgrind base

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  Apps
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android universal examples

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  Deploy
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documentation

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