NullPointerException in __computeSerializedValue

[mingyu89]

Hello, I encountered the same issue as this old one https://github.com/scalapb/ScalaPB/issues/249 with a similar stacktrace.

java.lang.NullPointerException: null
	at MyMessage.__computeSerializedValue(MyMessage.scala:130) ~[classes/:?]
	at MyMessage.serializedSize(MyMessage.scala:137) ~[classes/:?]
	at com.trueaccord.scalapb.GeneratedMessage.toByteArray(GeneratedMessageCompanion.scala:93) ~[scalapb-runtime_2.12-0.5.47.jar:0.5.47]
	at com.trueaccord.scalapb.GeneratedMessage.toByteArray$(GeneratedMessageCompanion.scala:92) ~[scalapb-runtime_2.12-0.5.47.jar:0.5.47]
	at MyMessage.toByteArray(MyMessage.scala:20) ~[classes/:?]

The generated code should check both null value and isDefined.

Steps to reproduce:

1. Create a proto message message1 with string field field1.
2. Serialize a message1 object o1 into byte array.
3. Update the proto message message1 and add a new string field field2
4. Deserialize object o1 using the update message1.