Style Engine: Bail early when adding a declaration if not passed a string

[andrewserong]

This PR hardens add_declaration in the Style Engine when passed non-string values. The changes are already in Gutenberg over in: WordPress/gutenberg#74881

Trac ticket: https://core.trac.wordpress.org/ticket/64545

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[tellthemachines]

Thanks for fixing this! LGTM.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61516
GitHub commit: b7cb91c

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.