Bluetooth: Audio: Shell: Fix compile warning for 0 client snk/src ASE

In case that CONFIG_BT_AUDIO_UNICAST_CLIENT_ASE_SNK_COUNT or
CONFIG_BT_AUDIO_UNICAST_CLIENT_ASE_SRC_COUNT is set to NULL, the
unicast client implementation in the shell would have compile
warnings, since the arrays would be 0 and we would attempt to
access it some places (or rather the compiler/linker would
think that, but it would never happen at runtime).

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