Fix embedded record set multi-select

[acwhite211]

Fixes #7793

Handle undefined field for isToOne. This fixed the issue of the embedded record set multi-select not working correctly.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list

Testing instructions

• Create a new record set on any table.
• Press the add button in the top left.
• Select query builder.
• Add any field.
• Press Query.
• Try to select multiple results .
• See Select button is enabled and can be pressed.

Working on test-panel https://blankdbsetup2elizabeth20260302-issue-7793.test.specifysystems.org/specify/view/collectingevent/1/?recordsetid=7

[bhumikaguptaa]

• See Select button is enabled and can be pressed.

It works as expected. I can add multiple records to the record set using select.

[emenslin]

• See Select button is enabled and can be pressed.

Looks good!