Skip scratch disks in disk delete checks (#16023)

Author: rileykarson

mmv1/templates/terraform/pre_delete/detach_disk.tmpl

@@ -112,7 +112,8 @@ if v, ok := readRes["users"].([]interface{}); ok {
 		if disks, ok := instanceRes["disks"].([]interface{}); ok {
 			for _, diskInterface := range disks {
 				disk := diskInterface.(map[string]interface{})
-				if tpgresource.CompareSelfLinkOrResourceName("", disk["source"].(string), self, nil) {
+				// source is nil for scratch disks
+				if source := disk["source"]; source != nil && tpgresource.CompareSelfLinkOrResourceName("", source.(string), self, nil) {
 					detachCalls = append(detachCalls, detachArgs{
 						project:    instanceProject,
 						zone:       tpgresource.GetResourceNameFromSelfLink(instanceRes["zone"].(string)),