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/gui/RadialNet.py

@@ -2042,10 +2042,7 @@ class NetNode(Node):
         if info == None:
             return self.__draw_info
 
-        if info in self.__draw_info:
-            return self.__draw_info[info]
-
-        return None
+        return self.__draw_info.get(info)
 
     def set_draw_info(self, info):
         """