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/zenmapGUI/DiffCompare.py

@@ -293,18 +293,12 @@ class ScanChooser(HIGVBox):
         """Return the currently selected scan's parsed output as an NmapParser
         object, or None if no valid scan is selected."""
         selected_scan = self.combo_scan.child.get_text()
-        if selected_scan in self.scan_dict:
-            return self.scan_dict[selected_scan]
-        # What's typed in the entry doesn't match a registered scan.
-        return None
+        return self.scan_dict.get(selected_scan)
 
     def get_nmap_output(self):
         """Return the currently selected scan's output as a string, or None if
         no valid scan is selected."""
-        parsed = self.parsed_scan
-        if parsed is not None:
-            return parsed.nmap_output
-        return None
+        return self.parsed_scan
 
     nmap_output = property(get_nmap_output)
     parsed_scan = property(get_parsed_scan)