As titled. The `Vec<UnshapedTextRun>` produced by `fn
build_unshaped_text_runs`
is only used by `into_shaped_text_run`, which filters out those without
string or font.
It is better to not allocate them in the first place.
Testing: [Try](https://github.com/servo/servo/actions/runs/23642089615).
Added a Servo-specific crash test to ensure we will not have empty
font/string in a text run.
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Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>