Fix missing default method for motion poll

[Elblinator]

Describe the bug If the default setting for the motion poll default voting methods was not set then no default is preselected in the dialog

How to Reproduce

1. Have a instance on version 4.1.15 or before
2. Create a meeting
3. Upgrade to 4.1.17.staging
4. Navigate to a motion detail view
5. Start creating a poll
6. The voting method will be empty, and you need to select a voting method
7. (The bug is fixed if you go into the settings and select a voting default method for motion polls)

Expected behavior The voting method yes/no/abstain should be preselected if the setting doesn't show something else

[luisa-beerboom]

It seems correct that there isn't a pre-selection in the client if the setting that is supposed to define the pre-selection is empty. The field motion_poll_default_method is missing a default, however, and there was no migration. Should that be changed?

It is no issue at all to add a default for the field and we can also add a migration that inserts the value YNA into the field on instances that don't have it set. After that, as far as I can understand it, the issue, as described, should be fixed.

Is there something I missed? Otherwise please move this to the backend.