Auto merge of #15299 - hiikezoe:empty-keyframe, r=emilio,pcwalton

Create KeyframeAnimation for empty keyframe rules

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We should create KeyframeAnimation even if css keyframe rule is empty because we should fire css animation events for such CSS animations.
Also we should ignore properties that are annotated with '!important'.  From the spec[1].
 using !important on them is invalid and will cause the property to be ignored

[1] https://drafts.csswg.org/css-animations-1/#keyframes
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15257

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