[FIX] Switch nullptr for 0 when assigning to a uint64_t

[zalo]

@hoffstadt Fixes this error in VS2019's compiler: C:\Repositories\DearPy3D\DearPy3D\renderer\mvGraphics.cpp(282,38): error C2440: '=': cannot convert from 'nullptr' to 'VkPipelineCache'

VS2019 appears to be complaining about nullptr because VkPipelineCache is typedef'd from a uint64_t and it doesn't like the conversion.