Fix `nrepl-switch-to-repl-buffer' to work with all connections

Commit 12558e8 breaks `nrepl-switch-to-repl-buffer` functionality by
assuming that all nrepl connections will be made using
`nrepl-jack-in`.

Using `nrepl-jack-in` sets the variable `nrepl-project-dir` to the
correct value, but this information cannot be retrieved when we
connect to an existing nRepl server using `nrepl-connect`. Since
`nrepl-project-dir` is unbound, the function `file-truename` throws a
null value exception.

This commit checks if `nrepl-project-dir` is set before calling
`file-truename`. Further, the message "nREPL connection not found"
does not make sense and thus should not be displayed.