Replace key existence tests with dict.get()

Replaced instances of this pattern:

if 'key' in somedict:
    var = somedict['key']
else:
    var = ""

...with this much simpler pattern:

var = somedict.get('key', "")

Some variations, like returning None if the key is not found were also
replaced.

zenmap/radialnet/core/XMLHandler.py

@@ -179,10 +179,7 @@ class XMLNode:
     def get_attr(self, attr):
         """
         """
-        if attr in self.__attrs:
-            return self.__attrs[attr]
-
-        return None
+        return self.__attrs.get(attr)
 
     def get_attrs(self):
         """