Check sensei_user_can_register_for_course filter when My Courses Page is not set

[JuanchoPestana]

Fixes #2858

Changes proposed in this Pull Request:

• Apply sensei_user_can_register_for_course filter when the "My courses page" is not set

The conditional statement in line 3091 is responsible for checking whether to show the "My courses" page as the registration form, or to show the default WordPress register form (the wp_register() inside the else on line 3097). The issue here is, that when entering that else on line 3097, the filter in discussion is not applied; causing the register button to appear.

Testing instructions:

1. Go to Settings > General and ensure the setting "Anyone can register" is set.
2. Go to Sensei LMS > Settings and ensure the "My Courses Page" setting is set to a page.
3. Add the code snippet: add_filter( 'sensei_user_can_register_for_course', '__return_false' );
4. In a logged-out window, visit the course. You should not see the Register button. This is the correct behaviour.
5. Now, go to Sensei LMS > Settings and unset the "My Courses Page" setting (i.e. set it to "Select a Page:").
6. In a logged-out window, visit the course. You should not see the Register button. This is the correct behaviour.

Proposed changelog entry for your changes:

• Apply sensei_user_can_register_for_course filter when the "My courses page" is not set.