MB-65809 Use default setting for plan stability if not present

In addition to UNSET of the entire plan_stability field for
system:settings, it is also possible to UNSET one of the
individual settings:

UPDATE system:settings UNSET plan_stability.error_policy;

In such cases we need to use default setting for the individual
setting as well.

Change-Id: I1a02824defe2581706481de9eb9d7597580a9267
Reviewed-on: https://review.couchbase.org/c/query/+/242515
Tested-by: Sitaram Vemulapalli <sitaram.vemulapalli@couchbase.com>
Reviewed-by: Sitaram Vemulapalli <sitaram.vemulapalli@couchbase.com>

Author: miaobingjie

settings/plan_stability_ee.go

@@ -77,6 +77,15 @@ func updatePlanStabilitySetting(requestId string, val interface{}) errors.Error
 	if !ok {
 		return errors.NewSettingsInvalidValue(PLAN_STABILITY, "map[string]interface{}", val)
 	}
+
+	// use default value if not present in the new settings (e.g. UNSET used)
+	if _, ok := psMap["mode"]; !ok {
+		psMap["mode"] = PS_MODE_OFF
+	}
+	if _, ok := psMap["error_policy"]; !ok {
+		psMap["error_policy"] = PS_ERROR_MODERATE
+	}
+
 	// GetPlanStabilitySetting() returns a copy of the settings
 	planStability, err := GetPlanStabilitySetting()
 	if err != nil {