Restore empty string default for conf.env['icu-libs']

Restore empty string default for conf.env['icu-libs']

The reworked ICU libraries configuration code used [] as
default for conf.env['icu-libs']. This breaks dependency analysis
in samba_deps.py because SAMBA_SUBSYSTEM() expects deps to be
a string.

Signed-off-by: Earl Chew <earl_chew@yahoo.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue May 14 14:44:06 UTC 2024 on atb-devel-224