placesview: Open location even if mount was not found

Some locations have to be mounted, but their mounts are not user-visible (e.g. smb-browse). Though this is maybe a bit weird, it is how it works for years. The problem is that the commit 267ea755, which tries to get the default location for opening, caused regression as it doesn't expect such possibility. Before this commit, such locations were opened without any issue, but nothing happens currently after clicking to "Connect" except of clearing the "Connect to Server" entry. Let's fallback to the original location if the mount was not found to fix this regression.

Fixes: nautilus#1811 (closed)