fix: missing def expense if no exp in first month

(cherry picked from commit 5c9ce575f6)

erpnext/accounts/deferred_revenue.py

@@ -360,8 +360,8 @@ def book_deferred_income_or_expense(doc, deferred_process, posting_date=None):
 			)
 
 		if not amount:
-			return
-
+			prev_posting_date = end_date
+		else:
 			gl_posting_date = end_date
 			prev_posting_date = None
 			# check if books nor frozen till endate: