handle typed nil pointers in encode

[yumosx]

What this PR does:

Which issue(s) this PR fixes:

Fixes https://github.com/apache/dubbo-go/issues/2517

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

handle typed nil pointers such as (*int32)(nil) by encoding them as hessian null

[wongoo]

@yumosx unit test TestMultipleLevelRecursiveDep2 failed, u can update the test and re-run the CI to check the reason:

	decoded := fmt.Sprintf("%v", obj)
	origin := fmt.Sprintf("%v", data)
	if decoded != origin {
		t.Errorf("deserialize mismatched, origin: %s, decoded: %s", origin, decoded)
	}

[yumosx]

Hi @wongoo,thanks for your review, this test seems correct. The CI failure was caused by another reason I will create a PR to fix it later.

[yumosx]

@wongoo @AlexStocks The CI failure issue has been fully resolved by https://github.com/apache/dubbo-go-hessian2/pull/387