[improvement] Beans with no fields generate toString methods returning constant values

[carterkozak]

After this PR

==COMMIT_MSG==
Beans with no fields generate toString methods returning constant values
==COMMIT_MSG==

[carterkozak]

Out of curiosity, @iamdanfox do you recall why this was added?

conjure-java/conjure-java-core/src/main/java/com/palantir/conjure/java/types/BeanGenerator.java

Lines 102 to 107 in 902a33e

	// Need to add JsonSerialize annotation which indicates that the empty bean serializer should be used to
	// serialize this class. Without this annotation no serializer will be set for this class, thus preventing
	// serialization.
	typeBuilder
	.addAnnotation(JsonSerialize.class)
	.addField(createSingletonField(objectClass));

Tests don't fail without it, but it may work differently on older builds of jackson.