ui: sync job: fix error if local namespace is selected first

ui: sync job: fix error if local namespace is selected first

When creating a new sync job and a local namespace is configured
without setting a remote first, the createMaxPrefixLength
was passed an array instead of a string/undefined/null, which
triggered a 'ns2.match is not a funtion exception', making the UI
glitchy afterwards.

Fixed by explicitly checking for a string. Verified that the other
user of NamespaceMaxDepthReduced, the prune job edit window, does not
break after the change.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>