Fixes incorrect data URI normalization.

[bobab12]

This fixes the incorrect normalization of data URIs in _wp_normalize_relative_css_links by skipping data URIs altogether.

Trac ticket: https://core.trac.wordpress.org/ticket/55177

[github-actions]

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[hellofromtonya]

Committed via changeset https://core.trac.wordpress.org/changeset/52754.