Fix default value for 'maasNodes' variable

PROD-33786

Change-Id: Ie61b6b3c808758e14d37e872a5f271a82788b9ab
diff --git a/backupninja-restore-pipeline.groovy b/backupninja-restore-pipeline.groovy
index a869fe3..5249072 100644
--- a/backupninja-restore-pipeline.groovy
+++ b/backupninja-restore-pipeline.groovy
@@ -2,7 +2,7 @@
 def salt = new com.mirantis.mk.Salt()
 def python = new com.mirantis.mk.Python()
 def pepperEnv = "pepperEnv"
-def maasNodes = {}
+def maasNodes = []
 def restoreSaltMasterAndMaas = (env.getProperty('RESTORE_SALTMASTER_AND_MAAS') ?: true).toBoolean()
 def restoreDogtag = (env.getProperty('RESTORE_DOGTAG') ?: true).toBoolean()