[GlobalStep] [iOS] "Add Tag" button should appear disabled/greyed out when the tag field is empty.

Environment: iPhone 6(v12.4.4), iPhone 11 Pro (v13.3.1).

Reproduction Rate: 4/4 100%

Steps To Reproduce:

1. Install and open the application.
2. log in with valid credentials.
3. Go to Reader
4. Tap the cog in the top corner
5. Tap the 'Add a Topic' option
6. Note that there is an empty field and "Add Tag" button.
7. Now tap on "Add Tag" button without entering any tag
8. Observe that "Add Tag" button fails to disable and user navigates back to Reader tab upon tapping "Add Tag" button.

Actual Result: [GlobalStep] [iOS] "Add Tag" button should appear disabled/greyed out when the tag field is empty.

Expected Result: "Add Tag" button should remain disabled when the tag field is empty or a validation "Please enter a tag" should be displayed upon tapping Add tag button.

#2229.zip