Fix PHP notice on contribution page

[mattwire]

Overview

When you have membership enabled and you want to switch to contributions you will get a PHP notice if you click "Save" because you can't enable the contributions block without disabling the membership block first and amount_block_is_active is not set.

Before

PHP notice

After

PHP notice gone

Technical Details

Comments

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

I think enotices in form rules don't require tests - it's 'formy'