Fixed crash when RunInSeriesFP's color input is nil

SeriesFragmentProcessor would call emitChild with "" input color as opposed to nil input color when ginven a nil input color.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2106893003

Review-Url: https://codereview.chromium.org/2106893003
1 file changed