Non-strings break internal search

[alycejenni]

The internal search mechanism used in e.g. multiselect inputs does not check if a value is a string before trying to convert it to lowercase.

https://github.com/NaturalHistoryMuseum/zoa/blob/dev/src/components/inputs/dropdown/ZoaMultiselect.vue#L282-L295

The easiest fix is probably just to cast to a string before running the match.