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>

modules/git/commit_submodule.go

@@ -16,7 +16,7 @@ func (c *Commit) GetSubModules() (*ObjectCache[*SubModule], error) {
 	entry, err := c.GetTreeEntryByPath(".gitmodules")
 	if err != nil {
 		if _, ok := err.(ErrNotExist); ok {
-			return nil, nil
+			return nil, nil //nolint:nilnil // return nil to indicate that the submodule does not exist
 		}
 		return nil, err
 	}
@@ -48,5 +48,5 @@ func (c *Commit) GetSubModule(entryName string) (*SubModule, error) {
 			return module, nil
 		}
 	}
-	return nil, nil
+	return nil, nil //nolint:nilnil // return nil to indicate that the submodule does not exist
 }