Fix C-j behaviour

In ido-mode C-j is bound to `ido-select-text`, which will open a new
buffer if a buffer with the given name does not exist.

Previously `ffir-ido-find-file` would call `find-file` with `nil` as the
argument if file was not found in file-list. This would result in an
error.

To reproduce:
1) Open a file in a repository
2) Run `find-file-in-repository`
3) Type the name of file that does not exist (e.g. `test`)
4) Press `C-j`

The following message appears in the minibuffer and Messages buffer:
`find-file-noselect: Wrong type argument: stringp, nil`

This PR fixes the issue by just passing the plain filename to find-file in case it's not found in file-list, which seems to work OK. This behaviour should be indentical to `ido-find-file`.