lib: explicitly support the case of empty input for completions

lib: explicitly support the case of empty input for completions

When the user tab- or ?-completes when the character prior to
the position of the cursor is a space, completion logic is
passed null. Explicitly handle this case instead of using
partly_match, which has special logic associated with it to
allow abbreviating certain tokens.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>