FileFilterHotspot: fix a couple of issues

- Handle `grep -n` output with long lines, where there is no space
  after "path/to/file:123:", we can send "path/to/file:123:bla" to
  a text editor and it seems most of them just ignore the text after
  the last ":"
- If the session pointer is null, there is nothing to do
- Guard against editorCmd not having any spaces in it, which it could
  be with the "Custom" entry (from the combobox)
- Even when the file URL doesn't match the /path/to/file:line:column
  pattern try to use the editor specified in the profile settings