fix: NoneType object has no attribute "gstin" (#16458)

* fix: NoneType object has no attribute "gstin"

* fix: handle NoneType values
diff --git a/erpnext/regional/india/utils.py b/erpnext/regional/india/utils.py
index fd0eb34..d7a0258 100644
--- a/erpnext/regional/india/utils.py
+++ b/erpnext/regional/india/utils.py
@@ -5,7 +5,7 @@
 from erpnext.controllers.taxes_and_totals import get_itemised_tax, get_itemised_taxable_amount
 
 def validate_gstin_for_india(doc, method):
-	if not hasattr(doc, 'gstin'):
+	if not hasattr(doc, 'gstin') or not doc.gstin:
 		return
 
 	doc.gstin = doc.gstin.upper().strip()