Fix behavior of json modifier to match what json.Unmarshal does for nil values

In the original implementation I thought that the `json` package
would set fields to its zero value during unmarshal if the input
JSON had a `null` value for that attribute.

After performing an small experiment I noticed this is not the case.

Thus, I have removed that behavior and now if the json contains
a `null` value for a field this field will be ignored by the
json modifier thus keeping its original value whatever it was.

internal/modifiers/json_modifier.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"reflect"
 )
 
 // This modifier serializes objects as JSON when
@@ -13,10 +12,6 @@ import (
 var jsonModifier = AttrModifier{
 	Scan: func(ctx context.Context, opInfo OpInfo, attrPtr interface{}, dbValue interface{}) error {
 		if dbValue == nil {
-			v := reflect.ValueOf(attrPtr)
-			// Set the struct to its 0 value just like json.Unmarshal
-			// does for nil attributes:
-			v.Elem().Set(reflect.Zero(reflect.TypeOf(attrPtr).Elem()))
 			return nil
 		}
 

internal/modifiers/json_modifier_test.go

@@ -21,9 +21,11 @@ func TestAttrScan(t *testing.T) {
 		expectErrToContain []string
 	}{
 		{
-			desc:          "should set struct to zero value if input is nil",
-			dbInput:       nil,
-			expectedValue: FakeAttr{},
+			desc:    "should not set struct to zero value if input is nil",
+			dbInput: nil,
+			expectedValue: FakeAttr{
+				Foo: "notZeroValue",
+			},
 		},
 		{
 			desc:    "should work when input is a byte slice",