fix: handle empty string as false in EvaluateBase class

expressionparser.py

@@ -71,6 +71,7 @@ class EvaluateBase(object):
         if value == 'false': return False
         if value == False: return False
         if value == None: return False
+        if value == '': return False
         # this should be handle explicitely by the condition
         # edit: this is actually usefull if don't want disable to be evaluated as true
         if value == 'unavailable': return False