FIX Don't make a count query when threshold = 0

[lekoala]

This is a partial fix for #11645

It avoids count queries when dropdown_field_threshold is set to 0

Everything will work the same as before, but with a very small difference: empty tables will now lazy load, but I think that's acceptable

It will also prevent setting a limit of 0

Description

See issue for context

Manual testing steps

Set the dropdown_field_threshold to 0
See that no more count queries are made during scaffolding

Issues

#11645

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[lekoala]

@GuySartorelli I also took the opportunity to avoid setting the limit of items if threshold is 0. I'm not even sure why the threshold value is used as the lazy load limit, as this seems two different things

[GuySartorelli]

LGTM