fix: make conferencing provider optional during batch creation

The conferencing_provider Select field defaulted to "Zoom" (first option)
when not explicitly set, causing batch creation to fail with
"Please select a Zoom account for this batch" since no zoom_account
is provided at creation time.

frontend/src/pages/Batches/BatchForm.vue

@@ -483,6 +483,10 @@ const trashBatch = (close) => {
 
 const conferencingOptions = computed(() => {
 	return [
+		{
+			label: '',
+			value: '',
+		},
 		{
 			label: 'Zoom',
 			value: 'Zoom',