ComposeMenu.js: Handle null for fileName in response from ImagePicker.

Fixes #3813.

On iOS, it appears that the fileName is not included in the response given
by ImagePicker.launchCamera if the user has not granted read access to their
photo library. The case where fileName is undefined was handled correctly,
but it turns out it's coming through as null, which was not handled. Neither
the types nor the docs from react-native-image-picker are clear on this point,
so that was filed as
react-native-image-picker/react-native-image-picker#1271.

I did not create a local copy of react-native-image-picker with the correct type,
since it's very likely that we'll need to upgrade to later versions frequently as
we're on an early version of this library which is known to have limitations
and bugs.

I added tests for fileName being undefined and being null.

src/compose/ComposeMenu.js

@@ -32,10 +32,10 @@ type Props = $ReadOnly<{|
  * actual format.  The clue we get in the image-picker response is the extension
  * found in `uri`.
  *
- * Also if `fileName` is undefined, default to the last component of `uri`.
+ * Also if `fileName` is null or undefined, default to the last component of `uri`.
  */
-export const chooseUploadImageFilename = (uri: string, fileName: string | void): string => {
-  const name = fileName !== undefined ? fileName : uri.replace(/.*\//, '');
+export const chooseUploadImageFilename = (uri: string, fileName: ?string): string => {
+  const name = fileName ?? uri.replace(/.*\//, '');
 
   /*
    * Photos in an iPhone's camera roll (taken since iOS 11) are typically in
@@ -52,7 +52,7 @@ export const chooseUploadImageFilename = (uri: string, fileName: string | void):
 };
 
 class ComposeMenu extends PureComponent<Props> {
-  uploadFile = (uri: string, fileName: string | void) => {
+  uploadFile = (uri: string, fileName: ?string) => {
     const { dispatch, destinationNarrow } = this.props;
     dispatch(uploadFile(destinationNarrow, uri, chooseUploadImageFilename(uri, fileName)));
   };
@@ -63,7 +63,7 @@ class ComposeMenu extends PureComponent<Props> {
     // https://github.com/react-native-community/react-native-image-picker/blob/master/docs/Reference.md
     error?: string | void | null | false,
     uri: string,
-    fileName: string,
+    fileName: ?string,
   }) => {
     if (response.didCancel) {
       return;

src/compose/__tests__/ComposeMenu-test.js

@@ -16,4 +16,14 @@ describe('chooseUploadImageFilename', () => {
       ).toBe(`${fileNameWithoutExtension}.jpeg`);
     },
   );
+
+  test('Uses the last component of uri if fileName is null', () => {
+    const fileName = null;
+    expect(chooseUploadImageFilename('some/path/something.jpg', fileName)).toBe('something.jpg');
+  });
+
+  test('Uses the last component of uri if fileName is undefined', () => {
+    const fileName = undefined;
+    expect(chooseUploadImageFilename('some/path/something.jpg', fileName)).toBe('something.jpg');
+  });
 });