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>

models/git/lfs_lock.go

@@ -130,7 +130,7 @@ func GetLFSLockByRepoID(ctx context.Context, repoID int64, page, pageSize int) (
 // GetTreePathLock returns LSF lock for the treePath
 func GetTreePathLock(ctx context.Context, repoID int64, treePath string) (*LFSLock, error) {
 	if !setting.LFS.StartServer {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when LFS is not started
 	}
 
 	locks, err := GetLFSLockByRepoID(ctx, repoID, 0, 0)
@@ -142,7 +142,7 @@ func GetTreePathLock(ctx context.Context, repoID int64, treePath string) (*LFSLo
 			return lock, nil
 		}
 	}
-	return nil, nil
+	return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 }
 
 // CountLFSLockByRepoID returns a count of all LFSLocks associated with a repository.

models/git/protected_branch.go

@@ -318,7 +318,7 @@ func GetProtectedBranchRuleByName(ctx context.Context, repoID int64, ruleName st
 	if err != nil {
 		return nil, err
 	} else if !exist {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return rel, nil
 }
@@ -329,7 +329,7 @@ func GetProtectedBranchRuleByID(ctx context.Context, repoID, ruleID int64) (*Pro
 	if err != nil {
 		return nil, err
 	} else if !exist {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return rel, nil
 }

models/git/protected_tag.go

@@ -104,7 +104,7 @@ func GetProtectedTagByID(ctx context.Context, id int64) (*ProtectedTag, error) {
 		return nil, err
 	}
 	if !has {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return tag, nil
 }
@@ -117,7 +117,7 @@ func GetProtectedTagByNamePattern(ctx context.Context, repoID int64, pattern str
 		return nil, err
 	}
 	if !has {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return tag, nil
 }