network: fix show wifi connection
Viewing a wifi connection will cause a 500 error. To reproduce this issue, go to plinth/sys/networks/. If no wifi connections exist, add one use the "Add Connection" button. Then, click on the name of the wifi connection to view it. A 500 error is returned.
This commit fixes this error so the normal "show connection" is returned. It also fixes a bytestring conversion error for the SSID and a blank value for "mode" on the page.