Bugfix no project selected on join crash

[maciejSamerdak]

Resolves #141

Done:

• If there are no projects available for joining, the form with project selection visible in popup is replaced with appropriate message, preventing user from submitting an empty form resulting in app crash

[rwrzesien]

if not project_form_queryset.exists()

[rwrzesien]

This solution seems OK, what I would just do is separate the message display logic from form creation - form still could be created with empty queryset (instead of returning None), but not displayed depending on different variable passed to template. This way we avoid tightly coupling those two elements.

I would also add some validation to the form itself, in case if someone tries to still manually call POST on this view, that checks if queryset is empty and then raise ValidationError. This way we will secure it in both frontend (message) and backend (validation).

[maciejSamerdak]

I introduced fixes that change the current implementation to one that's closer to what @rwrzesien suggested.