OtaDexoptService: Add missing return

Add early-return for null handling.

Bug: 38186355
Test: m
Test: manual OTA
Change-Id: I9076427ab0ddb97f2c81ff208947a0d172d1b650

services/core/java/com/android/server/pm/OtaDexoptService.java

@@ -316,6 +316,7 @@ public class OtaDexoptService extends IOtaDexopt.Stub {
 
                 if (arg == null) {
                     builder.append('!');
+                    return;
                 }
 
                 String txt = String.valueOf(arg);