Error when user has no group and 'Allow students to see the results' is enabled

[odeialba]

I could not replicate it, but this bug was reported:
This error happens only in the app. If you have a user without any group and you have the option 'Allow students to see the results' enabled, when the user submits a vote, there is an error saying: Coding error detected, it must be fixed by a programmer: moodle_database::get_in_or_equal() does not accept empty arrays
The error might come from trying to show the results from a group that doesn't exist.
If the user is in a group or if the options 'Allow students to see the results' is disabled, there is no error.

[odeialba]

I was able to "not see" the results for users without group, but still didn't get any error. But that was because the results that were being shown belonged to a group that didn't submit any vote. That problem is related to #66

[odeialba]

I was able to see the error in the console on Moodle 4.4dev