fields with "multiple" enabled save an empty value when nothing set

When a field is set to "multiple" and nothing gets set, an empty meta value gets inserted. Instead, I think no meta value should be inserted.

Start by reproducing the behavior for a custom field with “multiple” enabled while leaving it unset. Trace where saving handles the empty value, then verify that no meta value is inserted in that case while populated multiple fields continue to save correctly.

Written by the indexing model from the issue text.