fix: gstin validation should work when there is no state (#16378)

diff --git a/erpnext/regional/india/utils.py b/erpnext/regional/india/utils.py
index 9f161af..fd0eb34 100644
--- a/erpnext/regional/india/utils.py
+++ b/erpnext/regional/india/utils.py
@@ -21,7 +21,9 @@
 
 	validate_gstin_check_digit(doc.gstin)
 
-	if not doc.gst_state and doc.state:
+	if not doc.gst_state:
+		if not doc.state:
+			return
 		state = doc.state.lower()
 		states_lowercase = {s.lower():s for s in states}
 		if state in states_lowercase: