Return nil instead of "null"

A scenario is when FF disables Flash, BES returns "null", and when
modules try to use Gem::Version, the "null" is considered a malformed
data and it won't be able to continue.

lib/msf/core/exploit/remote/browser_exploit_server.rb

@@ -303,7 +303,7 @@ module Msf
         parsed_body = CGI::parse(Rex::Text.decode_base64(request.body) || '')
         vprint_status("Received sniffed browser data over POST:")
         vprint_line("#{parsed_body}.")
-        parsed_body.each { |k, v| profile[k.to_sym] = v.first }
+        parsed_body.each { |k, v| profile[k.to_sym] = (v.first == 'null' ? nil : v.first) }
         found_ua_name = parsed_body['ua_name']
         found_ua_ver = parsed_body['ua_ver']