Bluetooth: Audio: Fix issue with unicast client src/snk ASE count

If either the sink or source ASE count was zero, calls to
ARRAY_SIZE(srcs) or ARRAY_SIZE(sinks) would cause a build warning.

The arrays should actually not be there at all if the respective
ASE count was 0, as that is just a waste of memory. The arrays,
and all uses of them, have been properly guarded.

This also adds a build assert to ensure that at least one
of them is non-zero, and that we also test building either
of them with the value 0.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>