Add support for empty properties vector in commandNDRangeKernel

[MohammadBadir]

In the C API, we pass properties in a 0 terminated array, or we can pass nullptr if we don't want any properties.

In the C++ bindings, properties are passed in a vector reference. This forces us to pass a vector containing 0 if we don't want properties, causing boilerplate. My change allows passing an empty vector, which would pass nullptr to the C API.

[CLAassistant]

All committers have signed the CLA.

[bashbaug]

Thanks!

This is a bit different than the pattern used by other functions with vectors of properties, but I like this version better.