bool = pcontext:IsNull()
True if the context is NULL, i.e. invalid. If functions
that allocate contexts fail, they might not throw an error but simply set the
context to NULL. You can use this function to check if context allocation has
failed in which case the context will be NULL.
Also, certain getter functions can return a NULL object in case the object
doesn't exist. You can use this function to check if a getter function returned
a NULL handle.
True if the context is NULL, otherwise False