rpc: convert null value to a defaultT

Right now we convert it is unmarshaler into a float64(0)
so an error is supressed.

pkg/rpc/request/param.go

@@ -243,6 +243,11 @@ func (p *Param) UnmarshalJSON(data []byte) error {
 		{ArrayT, &[]Param{}},
 	}
 
+	if bytes.Equal(data, []byte("null")) {
+		p.Type = defaultT
+		return nil
+	}
+
 	for _, cur := range attempts {
 		r := bytes.NewReader(data)
 		jd := json.NewDecoder(r)

pkg/rpc/request/param_test.go

@@ -14,7 +14,7 @@ import (
 )
 
 func TestParam_UnmarshalJSON(t *testing.T) {
-	msg := `["str1", 123, ["str2", 3], [{"type": "String", "value": "jajaja"}],
+	msg := `["str1", 123, null, ["str2", 3], [{"type": "String", "value": "jajaja"}],
                  {"primary": 1},
                  {"sender": "f84d6a337fbc3d3a201d41da99e86b479e7a2554"},
                  {"cosigner": "f84d6a337fbc3d3a201d41da99e86b479e7a2554"},
@@ -36,6 +36,9 @@ func TestParam_UnmarshalJSON(t *testing.T) {
 			Type:  NumberT,
 			Value: 123,
 		},
+		{
+			Type: defaultT,
+		},
 		{
 			Type: ArrayT,
 			Value: []Param{