No fallback to English field label

[Zverik]

When some of the field strings are translated, others come back as empty. For example, stroller or toilets/wheelchair as of now: English strings have label, placeholder, options, but Russian translation has just options. The SQL query in providers/presets.dart:getFields() returns empty strings for label and placeholder. Naturally there are non-empty values down the line, but we take the first matching row and discard others. As a result, those fields in the form are named with their system field names.

This requires major SQL overhaul, so postponing.