notes: simplify default operation mode arguments check

notes: simplify default operation mode arguments check

'git notes' has a default operation mode, but when invoked without a
subcommand it doesn't accept any arguments (although the 'list'
subcommand implementing the default operation mode does accept
arguments).  The condition checking this ended up a bit awkward, so
let's make it clearer.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>