When valueField/textField are not explicitly set in opts, the auto-select
logic (line 1140) and nullable empty-option creation (lines 1144-1145) used
data[0][undefined] which returned undefined, causing:
- Single-option selects to show blank (auto-select failed)
- nullable empty options to have undefined keys
Now extracts vf/tf local variables with ||'value'/||'text' fallback at the
top of build_options(), used consistently throughout.
