external download manager - commandline improperly handles nullable strings

[gue5t gue5t]

To reproduce (with latest git, vala 0.20), enable the "external download manager - Commandline" extension. Then try to download the file <http://nxzobi.people.wm.edu/ext1.pdf> (click, then select "save" in the download dialog). The following errors are printed:

(midori4:31433): GLib-CRITICAL **: g_shell_quote: assertion `unquoted_string != NULL' failed

** (midori4:31433): CRITICAL **: string_replace: assertion `replacement != NULL' failed

** (midori4:31433): CRITICAL **: string_replace: assertion `self != NULL' failed

** (midori4:31433): CRITICAL **: string_replace: assertion `self != NULL' failed

(midori4:31433): GLib-CRITICAL **: g_spawn_command_line_async: assertion `command_line != NULL' failed

The attached patch fixes these warnings and makes the download manager run.