fix: Only calculate first_respone_time if SLA is set (#27789) (#27793)

Co-authored-by: Afshan <33727827+AfshanKhan@users.noreply.github.com>
(cherry picked from commit ad03eb2)

Co-authored-by: Ganga Manoj <ganga.manoj98@gmail.com>

erpnext/support/doctype/issue/issue.py

@@ -539,7 +539,7 @@ def get_time_in_timedelta(time):
 def set_first_response_time(communication, method):
 	if communication.get('reference_doctype') == "Issue":
 		issue = get_parent_doc(communication)
-		if is_first_response(issue):
+		if is_first_response(issue) and issue.service_level_agreement:
 			first_response_time = calculate_first_response_time(issue, get_datetime(issue.first_responded_on))
 			issue.db_set("first_response_time", first_response_time)