Fix NPE in PhoneUtils#makePstnPhoneAccountHandle

Bug: 271214683
Test: atest PhoneUtilsTest
Change-Id: If258288ba33931da1c551709c2e7655c0fca65bf
diff --git a/src/com/android/phone/PhoneUtils.java b/src/com/android/phone/PhoneUtils.java
index 4826d2b..0c8a9c7 100644
--- a/src/com/android/phone/PhoneUtils.java
+++ b/src/com/android/phone/PhoneUtils.java
@@ -703,8 +703,12 @@
     }
 
     public static PhoneAccountHandle makePstnPhoneAccountHandle(Phone phone) {
-        return makePstnPhoneAccountHandleWithPrefix(phone, "",
-                false, phone.getUserHandle());
+        if (phone == null) {
+            return null;
+        } else {
+            return makePstnPhoneAccountHandleWithPrefix(phone, "",
+                    false, phone.getUserHandle());
+        }
     }
 
     public static PhoneAccountHandle makePstnPhoneAccountHandleWithPrefix(