private void validateNullDefault() {
        if (!(nullableVersions().contains(versions))) {
            throw new RuntimeException("null cannot be the default for field " +
                name + ", because not all versions of this field are " +
                "nullable.");
        }
    }
Get a string representation of the field default.

@param headerGenerator   The header generator in case we need to add imports.
@param structRegistry    The struct registry in case we need to look up structs.

@return                  A string that can be used for the field default in the
                         generated code.
generator/src/main/java/org/apache/kafka/message/FieldSpec.java