Fix 'if' in the clear_isolated_creds

Change-Id: I7700db2ce71779e64daae2c787dbac28ce8bf613
diff --git a/tempest/tests/compute/base.py b/tempest/tests/compute/base.py
index f2af1fc..594535f 100644
--- a/tempest/tests/compute/base.py
+++ b/tempest/tests/compute/base.py
@@ -154,7 +154,7 @@
     @classmethod
     def clear_isolated_creds(cls):
         if not cls.isolated_creds:
-            pass
+            return
         admin_client = cls._get_identity_admin_client()
 
         for user, tenant in cls.isolated_creds:
diff --git a/tempest/tests/volume/base.py b/tempest/tests/volume/base.py
index 6625180..c91244f 100644
--- a/tempest/tests/volume/base.py
+++ b/tempest/tests/volume/base.py
@@ -113,7 +113,7 @@
     @classmethod
     def clear_isolated_creds(cls):
         if not cls.isolated_creds:
-            pass
+            return
         admin_client = cls._get_identity_admin_client()
 
         for user, tenant in cls.isolated_creds: