fix not compiling on certain windows versions

[chearon]

msbuild is throwing this in the prebuild workflow, and I give up trying to figure out why it's different than the main workflow.

Error: D:\a\node-canvas\node-canvas\src\register_font.cc(261,18): error C2664: 'HANDLE CreateFileW(LPCWSTR,DWORD,DWORD,LPSECURITY_ATTRIBUTES,DWORD,DWORD,HANDLE)': cannot convert argument 6 from 'nullptr' to 'DWORD' [D:\a\node-canvas\node-canvas\build\canvas.vcxproj]

The error makes sense, this arg should not be NULL.