[Kernel] Adds occa::null

[dmed256]

Description

#235 occa::null can be passed to kernels as a way to omit an occa::memory argument

[codecov]

Codecov Report

Merging #270 into master will increase coverage by 1.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
+ Coverage    71.7%   73.06%   +1.36%     
==========================================
  Files         282      282              
  Lines       19520    19532      +12     
==========================================
+ Hits        13996    14272     +276     
+ Misses       5524     5260     -264

Impacted Files	Coverage Δ
include/occa/core/kernelArg.hpp	100% <100%> (ø)	⬆️
src/core/kernel.cpp	91.41% <100%> (+0.04%)	⬆️
src/core/kernelArg.cpp	81.94% <100%> (+1.34%)	⬆️
src/c/kernel.cpp	1.36% <0%> (-95.9%)	⬇️
src/lang/expr/stringNode.cpp	0% <0%> (-55%)	⬇️
include/occa/tools/hash.hpp	83.33% <0%> (-16.67%)	⬇️
src/c/types.cpp	77.08% <0%> (-6.46%)	⬇️
src/tools/sys.cpp	69.18% <0%> (-5.68%)	⬇️
src/lang/specialMacros.cpp	59.32% <0%> (+0.84%)	⬆️
src/lang/token.cpp	75.11% <0%> (+1.33%)	⬆️
... and 31 more