Media: Move a variable definition outside of conditionals to ensure it's always available.

This fixes cases where the URL to a PDF preview may be incorrectly calculated when no thumbnails were generated for the PDF (and avoids a PHP Notice at the same time).

Fixes #39250.

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git-svn-id: https://core.svn.wordpress.org/trunk@39552 1a063a9b-81f0-0310-95a4-ce76da25c4cd