[hotfix] ignore if student email address field value is None (#11116)

diff --git a/erpnext/schools/doctype/fees/fees.py b/erpnext/schools/doctype/fees/fees.py
index a2fcecc..42b329e 100644
--- a/erpnext/schools/doctype/fees/fees.py
+++ b/erpnext/schools/doctype/fees/fees.py
@@ -50,6 +50,7 @@
 			select g.email_address
 			from `tabGuardian` g, `tabStudent Guardian` sg
 			where g.name = sg.guardian and sg.parent = %s and sg.parenttype = 'Student'
+			and ifnull(g.email_address, '')!=''
 		""", self.student)
 
 		student_email_id = frappe.db.get_value("Student", self.student, "student_email_id")