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

Co-authored-by: Afshan <33727827+AfshanKhan@users.noreply.github.com>
diff --git a/erpnext/support/doctype/issue/issue.py b/erpnext/support/doctype/issue/issue.py
index 7d7399d..0fe1068 100644
--- a/erpnext/support/doctype/issue/issue.py
+++ b/erpnext/support/doctype/issue/issue.py
@@ -228,7 +228,7 @@
 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)