Fix a few issues with the new bookmarks handling code

I noticed a few issues with the way bookmarks are handled since f96357d. For instance, clearing the bookmarks file manually without resetting the sidebar-bookmark-breakpoint value as well, would cause newly added bookmarks to remain hidden despite the "Bookmarks" section being shown in the sidebar. Moreover, this PR avoids a segmentation fault caused by a missing NULL check in the DND code.
I had planned to include a comment as to why I removed this call, but decided not to after all. If you change the value in dconf-editor to something silly like 1000, it'll trigger a call to `update_places()` which would then immediately reset the value in gsettings (and thus dconf-editor) to the bookmark count. That seemed like a strange way to do it, hence why I removed the gsettings update. I'm not sure which behavior is preferable.