Check if DEFAULT_GIT_URL exists

Change-Id: I796efb070b0bb3e92582703cf60b1de10a0e2c8a
diff --git a/src/com/mirantis/mk/SaltModelTesting.groovy b/src/com/mirantis/mk/SaltModelTesting.groovy
index 2bced6c..f5c66ee 100644
--- a/src/com/mirantis/mk/SaltModelTesting.groovy
+++ b/src/com/mirantis/mk/SaltModelTesting.groovy
@@ -46,7 +46,7 @@
       }
 
       def nodes
-      if (DEFAULT_GIT_URL.contains("mk-ci")) {
+      if (DEFAULT_GIT_URL && DEFAULT_GIT_URL.contains("mk-ci")) {
         nodes = sh script: "find /srv/salt/reclass/nodes -name '*.yml' | grep -v 'cfg*.yml'", returnStdout: true
       } else {
         nodes = sh script:"find /srv/salt/reclass/nodes/_generated -name '*.yml' | grep -v 'cfg*.yml'", returnStdout: true