Network/FormData typeof object allowing null

Summary:
Submitting PR in response to https://github.com/facebook/react-native/issues/12251

**Test plan (required)**

$formData = new FormData()
$formData.append('value', null)

Included Unit Tests
Closes https://github.com/facebook/react-native/pull/12253

Differential Revision: D4545291

Pulled By: lacker

fbshipit-source-id: 2780f24e1fe1e0d5be3bb12b64aa6cd9a20554bf

Libraries/Network/FormData.js

@@ -71,7 +71,7 @@ class FormData {
       // an object with a `uri` attribute. Optionally, it can also
       // have a `name` and `type` attribute to specify filename and
       // content type (cf. web Blob interface.)
-      if (typeof value === 'object') {
+      if (typeof value === 'object' && value) {
         if (typeof value.name === 'string') {
           headers['content-disposition'] += '; filename="' + value.name + '"';
         }

Libraries/Network/__tests__/FormData-test.js

@@ -0,0 +1,59 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+jest.unmock('FormData');
+
+const FormData = require('FormData');
+
+describe('FormData', function() {
+  var formData;
+
+  beforeEach(() => {
+    formData = new FormData();
+  });
+
+  afterEach(() => {
+    formData = null;
+  });
+
+  it('should return non blob null', function() {
+    formData.append('null', null);
+
+    const expectedPart = {
+      string: 'null',
+      headers: {
+        'content-disposition': 'form-data; name="null"'
+      },
+      fieldName: 'null'
+    };
+    expect(formData.getParts()[0]).toMatchObject(expectedPart);
+  });
+
+  it('should return blob', function() {
+    formData.append('photo', {
+      uri: 'arbitrary/path',
+      type: 'image/jpeg',
+      name: 'photo.jpg'
+    });
+
+    const expectedPart = {
+      uri: 'arbitrary/path',
+      type: 'image/jpeg',
+      name: 'photo.jpg',
+      headers: {
+        'content-disposition': 'form-data; name="photo"; filename="photo.jpg"',
+        'content-type': 'image/jpeg'
+      },
+      fieldName: 'photo'
+    };
+    expect(formData.getParts()[0]).toMatchObject(expectedPart);
+  });
+});