UI: Fix warning when compiling with Clang 10

Building with Clang 10 on FreeBSD 13-CURRENT emitted a warning about
logical not (!) applying only to the left hand side of a comparison.
Update the expression to match the style of other flags conditionals.
Verified that object code is the same.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f