include: cbprintf: Fix call to memcpy with null pointer

cbprintf_package_convert may invoke z_cbprintf_cpy with a null pointer
to buf and zero length to indicate a flush operation. This triggers
an error from the undefined behavior sanitizer due to memcpy being called
with a null src, which is undefined behavior according to the C standard.

This is avoided by exiting early in z_cbprintf_cpy when length is zero.

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>