encoding/json: revert "fix decoding of JSON null values"

Fixes #11912.
Fixes #11937.

This reverts commit 1a99ba55df.

Change-Id: I32b76053fdabc59f28ca5bedf1b15c0baa8afae1
Reviewed-on: https://go-review.googlesource.com/12893
Reviewed-by: Didier Spezia <didier.06@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/encoding/json/decode.go

@@ -358,7 +358,7 @@ func (d *decodeState) indirect(v reflect.Value, decodingNull bool) (Unmarshaler,
 		if v.IsNil() {
 			v.Set(reflect.New(v.Type().Elem()))
 		}
-		if v.Type().NumMethod() > 0 && !decodingNull {
+		if v.Type().NumMethod() > 0 {
 			if u, ok := v.Interface().(Unmarshaler); ok {
 				return u, nil, reflect.Value{}
 			}