cscli lapi register: no error if the credentials file does not exist (#4570)

The lapi command group's PersistentPreRunE loads the API client
credentials before subcommands such as register and status run, so
register failed with "no such file or directory" unless the file it
is meant to create already existed. Skip that requirement for register
when a credentials path is configured, keeping the strict behavior for
subcommands that read the LAPI.

Fixes #4237