Make sure GridState always outputs a JSON Object string

src/Forms/GridField/GridState.php

@@ -106,11 +106,8 @@ class GridState extends HiddenField
      */
     public function Value()
     {
-        if (!$this->data) {
+        $data = $this->data ? $this->data->getChangesArray() : [];
-            return json_encode([]);
+        return json_encode($data, JSON_FORCE_OBJECT);
-        }
-
-        return json_encode($this->data->getChangesArray());
     }
 
     /**

tests/php/Forms/GridField/GridStateTest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace SilverStripe\Forms\Tests\GridField;
+
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\Forms\GridField\GridField;
+use SilverStripe\Forms\GridField\GridState;
+
+class GridStateTest extends SapphireTest
+{
+
+    public function testValue()
+    {
+        $gridfield = new GridField('Test');
+
+        $state = new GridState($gridfield);
+        $this->assertEquals('{}', $state->Value(), 'GridState without any data has empty JSON object for Value');
+
+        $data = $state->getData();
+        $data->initDefaults(['Foo' => 'Bar']);
+
+        $this->assertEquals('{}', $state->Value(), 'GridState without change has empty JSON object for Value');
+
+        $data->Foo = 'Barrr';
+
+        $this->assertEquals(
+            '{"Foo":"Barrr"}',
+            $state->Value(),
+            'GridState with changes returns has a JSON object string for Value.'
+        );
+    }
+
+}