fix: handle null branch names in AutoModeService

- Updated branchName assignment to use nullish coalescing, ensuring that unassigned features are correctly set to null instead of an empty string. This change improves the handling of feature states during the update process.

apps/server/src/services/auto-mode-service.ts

@@ -551,7 +551,7 @@ export class AutoModeService {
 
       // Update running feature with actual worktree info
       tempRunningFeature.worktreePath = worktreePath;
-      tempRunningFeature.branchName = branchName;
+      tempRunningFeature.branchName = branchName ?? null;
 
       // Update feature status to in_progress
       await this.updateFeatureStatus(projectPath, featureId, "in_progress");