fix: disabled attribute fallback (#1984)

Co-authored-by: Dominik Pschenitschni <mail@celement.de>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/1984
Reviewed-by: konrad <k@knt.li>
Co-authored-by: Dominik Pschenitschni <dpschen@noreply.kolaente.de>
Co-committed-by: Dominik Pschenitschni <dpschen@noreply.kolaente.de>

src/components/input/fancycheckbox.vue

@@ -2,7 +2,7 @@
 	<div :class="{'is-disabled': disabled}" class="fancycheckbox">
 		<input
 			:checked="checked"
-			:disabled="disabled || null"
+			:disabled="disabled || undefined"
 			:id="checkBoxId"
 			@change="(event) => updateData(event.target.checked)"
 			type="checkbox"/>