Return an error when autocompleting taxons without a vocabulary

[jrdh]

Closes: #5866

Firstly, just for completeness, the reason this was breaking was because we were attempting to run an empty in clause. The difference between the two queries is that the second has values in it's in clause:

What those values are and where they come from I'm not sure of though, I'm also not sure why both autocomplete paths were being used at different times on the same page.

Either way, the fundamental issue is in the scratchpads_taxonomic_name_field_autocomplete function. So what do we do if there aren't any vocabularies set on the field? There are 3 choices as far as I can see:

1. Return an empty array as we know that nothing should match the query.
2. Miss out the in clause and search all taxonomies.
3. Return an error.

I chose to go with the 3rd choice but I'm happy to debate this. I definitely don't think we should do option 2, but option 1 is a possibility.

Input from @alycejenni, @PaulKiddle and @benscott welcome!

[PaulKiddle]

I can't reproduce the original bug but the code changes look fine to me

[Archilegt]

Is this related to taxon names now not filled in for edition? When manually editing a classification, the name to be edited is not appearing by default, which causes the user having to type it or copy it completely, even if the edit was just a minor edit like a one-letter misspelling. When the taxon in question is a quadrinomial, the burden quadruplicates. I would recommend to change the current behavior of the taxon editor for having the name pre-filled in the unit name fields.

[PaulKiddle]

@Archilegt This sounds like an unrelated bug, could you please open a new issues for this? If possible with a screenshot. Thanks.