Singular fields in Proto 3 are incorrectly treated as required

Wire seems to treat singular fields in Proto 3 schemas as if they are required, generating non-nullable setters / getters / etc.

This seems to conflict with the behaviour described in the protobuf language guide, which describes singular as being closer to optional than to required.

Specifically, I would expect that a singular field in a proto3 definition would generate a "setter" function that accepts null as input (clearing the value), and a non-nullable "getter" function that substitutes the appropriate default if no value is present in the serialised message.

Example:

syntax = "proto2";
message Proto2Format {
  optional string field_1 = 1;
  optional string field_2 = 2;
}

syntax = "proto3";
message Proto3Format {
  string field_1 = 1;
  optional string field_2 = 2;
}

fun proto2ToProto3() {
  val proto2Message = Proto2Format.Builder().field_1(null).field_2(null).build()
  val proto3Message = Proto3Format.ADAPTER.decode(Proto2Format.ADAPTER.encode(proto2Message))
  assertThat(proto3Message.field_1).isEqualTo("") // Default is used because value was not serialised
  assertThat(proto3Message.field_2).isNull
}

fun proto3ToProto2() {
  val proto3Message = Proto3Format.Builder().field_1("").field_2(null).build()
  val proto2Message = Proto2Format.ADAPTER.decode(Proto3Format.ADAPTER.encode(proto3Message))
  assertThat(proto2Message.field_1).isNull // Value was not serialised because it was the default
  assertThat(proto2Message.field_2).isNull
}

fun proto3() {
  val proto3Message = Proto3Format.Builder().field_1(null).field_2(null).build()
  assertThat(proto3Message.field_1).isEqualTo("") // Default is used
  assertThat(proto3Message.field_2).isNull
}

Currently, if we fail to supply a value for singular fields, we encounter exceptions like the following:

java.lang.NullPointerException: Parameter specified as non-null is null: method com.my.namespace.MyWireClass.<init>, parameter my_singular_field

No source file or test is named in the issue. Start by reproducing the Kotlin examples with the supplied proto2 and proto3 definitions, then trace Wire's proto3 singular-field code generation and encoding behavior. Done means singular fields accept null in builders and use the protobuf default when absent, while optional fields remain nullable.