videoaggregator: Don't configure NULL chroma-site/colorimetry
requested to merge slomo/gst-plugins-base:videoaggregator-dont-configure-null-chroma-site-colorimetry into master
If there's no known value in the best caps then the functions to convert them to strings will return NULL. Having the fields not in the caps is not a problem, having them with a NULL value however will cause negotiation failures.