testing/testcase: Set proper default software type

By default proxy checks if software type is None, then it's setting it to
RootSoftwareInstance, but if it comes as "" (empty string) it's kept as is.

So for consistency with proxy set default software type, so that proxy can
manage it correctly.

slapos/testing/testcase.py

@@ -554,12 +554,12 @@ class SlapOSInstanceTestCase(unittest.TestCase):
 
   @classmethod
   def getInstanceSoftwareType(cls):
-    """Return software type for instance, default "".
+    """Return software type for instance, default None.
 
     To be defined by subclasses if they need to request instance with specific
     software type.
     """
-    return ""
+    return None
 
   # Unittest methods
   @classmethod