Return empty slice for associations with 0 results

When using `Preload` to include the results of a "has many"
relationship, Gorm previously returned an uninitialized slice for any
such relations that bore zero records. This distinction was most
apparent when the results were marshalled to a JSON representation--a
record with zero related records would be represented with `null`.

For example, consider the following schema:

    id | name
    ---|------
    1  | Lorin
    2  | Sue

    id | p_id | value
    ---|------|-------------------
    1  | 1    | lorin@example.com
    2  | 1    | lorin2@example.com

Querying with:

    db.Preload("Email").Find(&people)

And marshalling the resulting value of `people` to JSON would yield the
following string:

    [
      {
        "name": "Lorin",
        "email": [
          "lorin@example.com",
          "lorin2@example.com"
        ]
      },
      {
        "name": "Sue",
        "email": null
      }
    ]

Beyond being inconsistent, the value `null` in this response differs
semantically from the actual state of the database. The database
actually has zero related records for the second user, so a JSON value
of `[]` is appropriate.

Update the callback that processes "has many" relationships to
communicate empty query results with an empty slice.

callback_query_preload.go

@@ -186,9 +186,11 @@ func (scope *Scope) handleHasManyPreload(field *Field, conditions []interface{})
 		for j := 0; j < indirectScopeValue.Len(); j++ {
 			object := indirect(indirectScopeValue.Index(j))
 			objectRealValue := getValueFromFields(object, relation.AssociationForeignFieldNames)
-			if results, ok := preloadMap[toString(objectRealValue)]; ok {
 			f := object.FieldByName(field.Name)
+			if results, ok := preloadMap[toString(objectRealValue)]; ok {
 				f.Set(reflect.Append(f, results...))
+			} else {
+				f.Set(reflect.MakeSlice(f.Type(), 0, 0))
 			}
 		}
 	} else {

preload_test.go

@@ -90,6 +90,8 @@ func TestPreload(t *testing.T) {
 			}
 		} else if len(user.Emails) != 0 {
 			t.Errorf("should not preload any emails for other users when with condition")
+		} else if user.Emails == nil {
+			t.Errorf("should return an empty slice to indicate zero results")
 		}
 	}
 }
@@ -592,8 +594,14 @@ func TestNestedPreload9(t *testing.T) {
 		},
 		Level2_1: Level2_1{
 			Level1s: []Level1{
-				{Value: "value3-3"},
-				{Value: "value4-4"},
+				{
+					Value: "value3-3",
+					Level0s: []Level0{},
+				},
+				{
+					Value: "value4-4",
+					Level0s: []Level0{},
+				},
 			},
 		},
 	}
@@ -657,6 +665,7 @@ func TestNestedPreload10(t *testing.T) {
 		},
 		{
 			Value: "bar 2",
+			LevelA3s: []*LevelA3{},
 		},
 	}
 	for _, levelA2 := range want {