* fix(highlight): render Astro diffs containing empty expressions
Empty Astro expressions produced zero-length nested JavaScript ranges, which rejected the whole diff. Leave those expressions in the markup tree and preserve highlighting around them.
* test(highlight): make empty Astro expression assertions explicit
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Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>
An Astro { } expression containing a template string inside ${} closed at
the first } in that nested string, leaving the rest of the expression
uncolored. Astro and Vue now share one scanner for embedded JavaScript,
which already skips template substitutions.
Closes#6209
Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>