Wrapper types do not unwrap to `null` when accessed on unset fields

[TristonianJones]

When a wrapper type field like google.protobuf.StringValue is not set and accessed on a protobuf message field, the evaluator should return a null value for the field to be consistent with the spec, and with wrapper behavior in protobuf generated code.

message Example {
   google.protobuf.Int32Value priority = 1;
   google.protobuf.StringValue message = 2;
}

The following should evaluate to true.

// example set to Example{}
example.priority != null || example.message == null

Note, this is a semantic change, and comparisons with null are painful to use in CEL without heterogeneous equality support.