Fix issue with smithy union serialization in multiple protocols

[landonxjames]

#4306 fixes a bug in XmlBindingTraitSerializerGenerator.kt exposed by a change to the S3-Control model .
The model change introduced a smithy union type that contained a unit struct as a member. This generated a match arm like:

crate::types::ObjectEncryptionFilter::Sses3(inner) =>
{
    scope_writer.start_el("SSE-S3").finish();
}

inner is never used since there is nothing to serialize for a unit struct, so this caused a warning and CI failure.

A quick review of other ${protocol}SerializerGenerator classes shows that they likely also contain the same bug:

• CBOR:

  smithy-rs/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/CborSerializerGenerator.kt

  Lines 551 to 555 in b570622

  	if (member.isTargetUnit()) {
  	symbolProvider.toMemberName(member)
  	} else {
  	"${symbolProvider.toMemberName(member)}(inner)"
  	}

• JSON:

  smithy-rs/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/JsonSerializerGenerator.kt

  Lines 559 to 562 in b570622

  	if (member.isTargetUnit()) {
  	"${symbolProvider.toMemberName(member)}"
  	} else {
  	"${symbolProvider.toMemberName(member)}(inner)"

• Query:

  smithy-rs/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/serialize/QuerySerializerGenerator.kt

  Lines 361 to 365 in b570622

  	if (member.isTargetUnit()) {
  	"${symbolProvider.toMemberName(member)}"
  	} else {
  	"${symbolProvider.toMemberName(member)}(inner)"
  	}

We should fix this for all of our protocols and add some codegen tests for it (the updated S3-control model implicitly tests it for XML, but explicit tests would be better).