[css-highlight-api] HighlightsFromPoint: Not dealing with invalid static ranges

[jnjaeschke]

The spec creates temporary ranges for StaticRanges because StaticRanges currently don't support getClientRect(). However, this step assumes that creating a temp range succeeds, and also does not cover the case where an underlying StaticRange may be invalid.

cc @ffiori @stephanieyzhang @smaug----

[ffiori]

PR #12215 resolves this issue, let me know if there's any inconsistencies left.