lib: cmsis_rtos_v2: Handle possible zero length array

Change how we handle the case of
CONFIG_CMSIS_V2_THREAD_DYNAMIC_MAX_COUNT=0 so that we don't create
a zero length array.  Instead we can just ifdef out the code associated
with handling a dynamic stack allocation.

Fixes #28397

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>