bug: omitempy regard non-empty-type interface as empty

[AsterDY]

Reproducible codes

type OmitEmptyInterface struct {
    ErrCode int32       `json:"code"`
    Data    interface{} `json:"data,omitempty"`
}

func TestOmitEmptyInterface(t *testing.T) {
	// non-enmpty type
	var data *string
    resp := &OmitEmptyInterface{
       ErrCode: 123,
       Data:    data,
    }
    eout, eerr := json.Marshal(resp)
	sout, serr := sonic.Marshal(resp)
	require.Equal(t, eerr == nil, serr == nil)
	require.Equal(t, string(eout), string(sout))

	// empty type and value
	resp = &OmitEmptyInterface{
		ErrCode: 123,
		Data:    nil,
	 }
	 eout, eerr = json.Marshal(resp)
	 sout, serr = sonic.Marshal(resp)
	 require.Equal(t, eerr == nil, serr == nil)
	 require.Equal(t, string(eout), string(sout))
}

output

--- FAIL: TestOmitEmptyInterface (0.00s)
    /Users/admin/GOPATH/work/sonic/issue_test/issuexx_test.go:40: 
        	Error Trace:	/Users/admin/GOPATH/work/sonic/issue_test/issuexx_test.go:40
        	Error:      	Not equal: 
        	            	expected: "{\"code\":123,\"data\":null}"
        	            	actual  : "{\"code\":123}"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-{"code":123,"data":null}
        	            	+{"code":123}
        	Test:       	TestOmitEmptyInterface

Explaination

sonic check if a interface is empty using second field emptyInterface.word, while std lib using first field emptyInterface.typ

[AsterDY]

#540