Cherry-pick: Do not create incomplete DlColorSource objects from Skia gradients (#33131)

[flar]

Resolve merge conflicts for flutter/flutter#103173

[flutter-dashboard]

This pull request was opened against a branch other than main. Since Flutter pull requests should not normally be opened against branches other than main, I have changed the base to main. If this was intended, you may modify the base back to flutter-2.13-candidate.0. See the Release Process for information about how other branches get updated.

Reviewers: Use caution before merging pull requests to branches other than main, unless this is an intentional hotfix/cherrypick.

[flar]

I specified the release branch when I created this PR, but the UI seems to have forgotten it while creating the PR and running the tests so I just reset it again to flutter-2.13-candidate.0 Hopefully it will stick this time.

[skia-gold]

Gold has detected about 19 new digest(s) on patchset 1.
View them at https://flutter-engine-gold.skia.org/cl/github/33462

[XilaiZhang]

Thank you! seems like there are some tests failing. Are these tests okay to ignore or maybe we need to fix them? cc @CaseyHillers @itsjustkevin