Allow setting view_dispatcher callbacks to NULL again

[xMasterX]

What's new

• Allow setting view_dispatcher callbacks to NULL again
• Checks for callback existence is already present in the code
• Recently added furi_check removes ability to remove callback without freeing whole view_dispatcher, which breaks logic in some Apps

Verification

• Check that view_dispatcher_set_custom_event_callback(view_dispatcher, NULL) can be called in apps now again
• All working fine

Checklist (For Reviewer)

• PR has description of feature/bug or link to Confluence/Jira task
• Description contains actions to verify feature/bugfix
• I've built this code, uploaded it to the device and verified feature/bugfix

[skotopes]

@xMasterX I've checked flipper-xremote source code and it's kinda puzzling, clearly not how we want people to use GUI.
I'm going to accept this PR, but I still don't see valid use case for changing callbacks on fly.