drivers: wifi: nrf_wifi: add NULL check in parse_sband

Ensure event is not NULL before accessing its members in
nrf_wifi_parse_sband().

This adds a defensive check to avoid potential null dereference and ensures
the function returns early when event data is invalid (i.e., zero channels
or bitrates).

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>