Signup Epilogue: updating HUD shows when password field was cleared

Continuation of https://github.com/wordpress-mobile/WordPress-iOS/issues/9121

Expected behavior

The 'Changing password' HUD should not appear if I type & remove password.

Actual behavior

The 'Changing password' HUD appears if I type & remove password.

Steps to reproduce the behavior

1. Sign up for a new account via 'Sign up with Email'.
2. On the epilogue screen, tap on the password field.
3. Type something
4. Clear that field completely
5. Tap somewhere to dismiss the keyboard.
6. Press 'Continue'. Notice the 'Changing password' HUD appears.

Tested on iPhone SE, iOS 11.3, WPiOS 9.9.0.20180423