OtaDexoptService: Add missing return

Add early-return for null handling.

Bug: 38186355
Test: m
Test: manual OTA
Change-Id: I9076427ab0ddb97f2c81ff208947a0d172d1b650
diff --git a/services/core/java/com/android/server/pm/OtaDexoptService.java b/services/core/java/com/android/server/pm/OtaDexoptService.java
index 2e4b49a..271045a 100644
--- a/services/core/java/com/android/server/pm/OtaDexoptService.java
+++ b/services/core/java/com/android/server/pm/OtaDexoptService.java
@@ -316,6 +316,7 @@
 
                 if (arg == null) {
                     builder.append('!');
+                    return;
                 }
 
                 String txt = String.valueOf(arg);