Fixes #31194.

Summary of Changes

Fixes validation of required checkboxes.

Testing Instructions

node build/build.js --compile-js required.

Add a checkboxes field with required attribute to some form. E.g. edit plugins/editors/codemirror/codemirror.xml to add required="true" attribute to fullScreenMod field:

<field
	name="fullScreenMod"
	type="checkboxes"
	label="PLG_CODEMIRROR_FIELD_FULLSCREEN_MOD_LABEL"
	required="true"
	>
	<option value="Shift">PLG_CODEMIRROR_FIELD_VALUE_FULLSCREEN_MOD_SHIFT</option>
	<option value="Cmd">PLG_CODEMIRROR_FIELD_VALUE_FULLSCREEN_MOD_CMD</option>
	<option value="Ctrl">PLG_CODEMIRROR_FIELD_VALUE_FULLSCREEN_MOD_CTRL</option>
	<option value="Alt">PLG_CODEMIRROR_FIELD_VALUE_FULLSCREEN_MOD_ALT</option>
</field>

Edit Editors - CodeMirror plugin.
Try to save it with no options selected in Toggle Fullscreen Modifier field.

Actual result BEFORE applying this Pull Request

Form is submitted.

Expected result AFTER applying this Pull Request

Form is not submitted, message is shown:

Error
The form cannot be submitted as it's missing required data.
Please correct the marked fields and try again.

Documentation Changes Required

No.