Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.golangci.yml

@@ -18,6 +18,7 @@ linters:
     - mirror
     - modernize
     - nakedret
+    - nilnil
     - nolintlint
     - perfsprint
     - revive