code: private void appendOptionalArgString(StringBuilder sb, JSType paramType, boolean forAnnotations) {
      if (paramType.isUnionType()) {
        // Remove the optionality from the var arg.
        paramType =
            paramType
                .toMaybeUnionType()
                .getRestrictedUnion(registry.getNativeType(JSTypeNative.VOID_TYPE));
      }
      paramType.appendAsNonNull(sb, forAnnotations).append("=");
    }
docs: Gets the string representation of an optional param.