fix null encoding for nil pointer

[ahaostudy]

What this PR does:

In the previous implementation, dubbo-go-hessian2 would encode nil values of a specified type as empty bytes.

For example:

var null *int = nil

e := NewEncoder()
e.Encode(null)

len(e.buffer) == 0  // true

However, the expected encoding for nil values should be []byte("N"), regardless of the type of nil value.

This PR addresses the above issue. It encodes nil values of any type as []byte("N").

To avoid impacting existing code, this feature is designed as an optional option.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6c40744) 69.23% compared to head (1e5ed7c) 69.32%.

❗ Current head 1e5ed7c differs from pull request most recent head ebf53db. Consider uploading reports for the commit ebf53db to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #368      +/-   ##
==========================================
+ Coverage   69.23%   69.32%   +0.09%     
==========================================
  Files          28       28              
  Lines        3146     3149       +3     
==========================================
+ Hits         2178     2183       +5     
+ Misses        745      744       -1     
+ Partials      223      222       -1     

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[wongoo]

@ahaostudy the checking nil is not implemented completely, I think we can fix it, and not need to add a special config for it.

I change the code as follow, and it works. You can update this PR, and then I will approve it.

[ahaostudy]

@ahaostudy the checking nil is not implemented completely, I think we can fix it, and not need to add a special config for it.

I change the code as follow, and it works. You can update this PR, and then I will approve it.

In this implementation, support for encoding nil values of types *bool and *int32 is still not available.

[tiltwind]

ref #369

[wongoo]

LGTM