fix: incorrect available_qty being set (#22539)

* fix: incorrect available_qty being set

* style: descriptive variables
diff --git a/erpnext/public/js/utils/serial_no_batch_selector.js b/erpnext/public/js/utils/serial_no_batch_selector.js
index 42f9cabc..d9f6e1d 100644
--- a/erpnext/public/js/utils/serial_no_batch_selector.js
+++ b/erpnext/public/js/utils/serial_no_batch_selector.js
@@ -338,8 +338,8 @@
 							};
 						},
 						change: function () {
-							let val = this.get_value();
-							if (val.length === 0) {
+							const batch_no = this.get_value();
+							if (!batch_no) {
 								this.grid_row.on_grid_fields_dict
 									.available_qty.set_value(0);
 								return;
@@ -359,14 +359,11 @@
 								return;
 							}
 
-							let batch_number = me.item.batch_no ||
-								this.grid_row.on_grid_fields_dict.batch_no.get_value();
-
 							if (me.warehouse_details.name) {
 								frappe.call({
 									method: 'erpnext.stock.doctype.batch.batch.get_batch_qty',
 									args: {
-										batch_no: batch_number,
+										batch_no,
 										warehouse: me.warehouse_details.name,
 										item_code: me.item_code
 									},