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
 }

modules/git/foreachref/parser.go

@@ -100,7 +100,7 @@ func (p *Parser) Err() error {
 func (p *Parser) parseRef(refBlock string) (map[string]string, error) {
 	if refBlock == "" {
 		// must be at EOF
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to signal EOF
 	}
 
 	fieldValues := make(map[string]string)

modules/git/last_commit_cache.go

@@ -55,12 +55,12 @@ func (c *LastCommitCache) Put(ref, entryPath, commitID string) error {
 // Get gets the last commit information by commit id and entry path
 func (c *LastCommitCache) Get(ref, entryPath string) (*Commit, error) {
 	if c == nil || c.cache == nil {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when cache is not available
 	}
 
 	commitID, ok := c.cache.Get(getCacheKey(c.repoPath, ref, entryPath))
 	if !ok || commitID == "" {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when cache miss
 	}
 
 	log.Debug("LastCommitCache hit level 1: [%s:%s:%s]", ref, entryPath, commitID)

modules/git/log_name_status.go

@@ -106,7 +106,7 @@ func (g *LogNameStatusRepoParser) Next(treepath string, paths2ids map[string]int
 			case bufio.ErrBufferFull:
 				g.buffull = true
 			case io.EOF:
-				return nil, nil
+				return nil, nil //nolint:nilnil // return nil to signal EOF
 			default:
 				return nil, err
 			}
@@ -121,7 +121,7 @@ func (g *LogNameStatusRepoParser) Next(treepath string, paths2ids map[string]int
 			case bufio.ErrBufferFull:
 				g.buffull = true
 			case io.EOF:
-				return nil, nil
+				return nil, nil //nolint:nilnil // return nil to signal EOF
 			default:
 				return nil, err
 			}