staticd: fix warning when creating routes without SR-TE colors

staticd: fix warning when creating routes without SR-TE colors

The SR-TE color YANG leaf is optional so it shouldn't be created
unconditionally (it doesn't have a default value).

Fixes warnings like this when routes are created without specifying
a SR-TE color:
STATIC: libyang: Invalid value "" in "srte-color" element.
(/frr-routing:routing/control-plane-protocols/control-plane-protocol[type='frr-s
taticd:staticd'][name='staticd'][vrf='default']/frr-staticd:staticd/route-list[p
refix='99.0.0.1/32'][afi-safi='frr-routing:ipv4-unicast']/path-list[distance='1'
]/frr-nexthops/nexthop[nh-type='ip4'][vrf='default'][gateway='192.168.1.2'][inte
rface='(null)']/srte-color)

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>