Enable clang-tidy modernize-unary-static-assert

Official docs:

  The check diagnoses any static_assert declaration with an empty string
  literal and provides a fix-it to replace the declaration with a
  single-argument static_assert declaration.

.clang-tidy

@@ -100,7 +100,6 @@ Checks: '*,
     -modernize-replace-auto-ptr,
     -modernize-replace-disallow-copy-and-assign-macro,
     -modernize-return-braced-init-list,
-    -modernize-unary-static-assert,
     -modernize-use-auto,
     -modernize-use-default-member-init,
     -modernize-use-emplace,