[Data collection] "Other" multiple choice option with no text not saved

[gino-m]

The UI appears to be removing the value, resulting in google/ground-platform#2099 and google/ground-platform#2098.

We may need to add a bool field to MultipleChoiceResponses to indicate the "other" option was selected; we can't rely on other_text being set since proto3 does not have a separate "unset" state for optional fields, so an empty string is considered the same as an unset field.

@kenstershiro @anandwana001 FYI