init exe as nil instead of ''

Author: kernelsmith

lib/msf/core/exploit/exe.rb

@@ -42,7 +42,7 @@ def get_custom_exe(path = nil)
     path ||= datastore['EXE::Custom']
     print_status("Using custom payload #{path}, RHOST and RPORT settings will be ignored!")
     datastore['DisablePayloadHandler'] = true
-    exe = ''
+    exe = nil
     ::File.open(path,'rb') {|f| exe = f.read(f.stat.size)}
     exe
   end