luci-app-samba4: prevent calling split() on array or null values

Since `uci.get()` may return null or array values, we cannot blindly call
split() on the result. Use the safe `L.toArray()` helper which is intended
to deal with such situations.

Also clean up whitespace while we're at.

Fixes: #5080
Signed-off-by: Jo-Philipp Wich <jo@mein.io>