Unable to send empty JSON object with `@Body`

Both approaches don't seem to work using feign 12.3 with okhttp and GSON encoder/decoder:

@Headers({"Accept: application/json", "Content-Type: application/json"})
public interface Api {

    @RequestLine("POST /empty")
    @Body("%7B%7D")
    void postEncoded();

    @RequestLine("POST /empty")
    @Body("{}")
    void postUnencoded();
}

The first sends it verbatim, which can't be parsed by the server, and the second triggers the expansion logic, creating an exception:

java.lang.IllegalArgumentException: an expression is required.
	at feign.template.Expressions.create(Expressions.java:68) ~[feign-core-12.3.jar:?]
	at feign.template.Template.parseFragment(Template.java:218) ~[feign-core-12.3.jar:?]
	at feign.template.Template.parseTemplate(Template.java:202) ~[feign-core-12.3.jar:?]
	at feign.template.Template.<init>(Template.java:61) ~[feign-core-12.3.jar:?]
	at feign.template.BodyTemplate.<init>(BodyTemplate.java:54) ~[feign-core-12.3.jar:?]
	at feign.template.BodyTemplate.create(BodyTemplate.java:50) ~[feign-core-12.3.jar:?]
	at feign.RequestTemplate.bodyTemplate(RequestTemplate.java:909) ~[feign-core-12.3.jar:?]
	at feign.Contract$Default.lambda$new$2(Contract.java:285) ~[feign-core-12.3.jar:?]
	at feign.DeclarativeContract$GuardedAnnotationProcessor.process(DeclarativeContract.java:253) ~[feign-core-12.3.jar:?]
	at feign.DeclarativeContract.lambda$processAnnotationOnMethod$7(DeclarativeContract.java:93) ~[feign-core-12.3.jar:?]
	at java.util.ArrayList.forEach(ArrayList.java:1511) ~[?:?]
	at feign.DeclarativeContract.processAnnotationOnMethod(DeclarativeContract.java:93) ~[feign-core-12.3.jar:?]
	at feign.Contract$BaseContract.parseAndValidateMetadata(Contract.java:110) ~[feign-core-12.3.jar:?]
	at feign.Contract$BaseContract.parseAndValidateMetadata(Contract.java:65) ~[feign-core-12.3.jar:?]
	at feign.DeclarativeContract.parseAndValidateMetadata(DeclarativeContract.java:38) ~[feign-core-12.3.jar:?]
	at feign.ReflectiveFeign$ParseHandlersByName.apply(ReflectiveFeign.java:134) ~[feign-core-12.3.jar:?]
	at feign.ReflectiveFeign.newInstance(ReflectiveFeign.java:56) ~[feign-core-12.3.jar:?]
	at feign.ReflectiveFeign.newInstance(ReflectiveFeign.java:48) ~[feign-core-12.3.jar:?]
	at feign.Feign$Builder.target(Feign.java:196) ~[feign-core-12.3.jar:?]
	at feign.Feign$Builder.target(Feign.java:192) ~[feign-core-12.3.jar:?]

Start at the @Body processing path named in the trace, especially BodyTemplate, Template, Expressions, and RequestTemplate.bodyTemplate. Reproduce the two annotations with Feign 12.3, then add coverage for sending an empty JSON object without triggering expression expansion; done means the request body is {} and no IllegalArgumentException occurs.