intel/fs: Don't allocate a param array for zero push constants

intel/fs: Don't allocate a param array for zero push constants

Thanks to the ralloc invariant of "any pointer returned from ralloc can
be used as a context", calling ralloc_size with a size of zero will
cause it to allocate at least a header.  If we don't have any push
constants, then NULL is perfectly acceptable (and even preferred).

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>