Gson serialize null issue

[zhanglc]

the serialize null should be false when set spring.gson.serialize-nulls=false

fix issue: #13622

test case:

        @Test
	public void serializeNullsFalse() {
		this.contextRunner.withPropertyValues("spring.gson.serialize-nulls:false")
				.run((context) -> {
					Gson gson = context.getBean(Gson.class);
					assertThat(gson.serializeNulls()).isFalse();
				});
	}