nil encoding not correct

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

For example:
```go
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.

**What you expected to happen**:
It encodes nil values of any type as []byte("N").

**How to reproduce it (as minimally and precisely as possible)**:

**Anything else we need to know?**:

Start at the NewEncoder and Encode entry points and reproduce the nil pointer example from the issue. Trace how nil values are written and verify the encoder output for nil values of different specified types. Done means every nil value encodes as the single byte "N" rather than an empty buffer.

Written by the indexing model from the issue text.