Bluetooth: GATT: Fix not resetting CCC storage

If there are no CCC to be stored the value should be set to NULL so it
is properly cleared otherwise calling settings_str_from_bytes will leave
str uninitialized which may cause a crash when attempting to load the
value.

Fixes #11564

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>