Render plot type input type should be `object`, not `ImgData | None`

[schloerke]

Introduced in #621

Previous: da69ea0#diff-3848a3fa2e11daf6f4de6a2be47908128df1ca342c8da4ccfc3692ecc2d53c88L190

RenderPlotFunc = Callable[[], object]
RenderPlotFuncAsync = Callable[[], Awaitable[object]]

which defines IT as object.

Current:

async def PlotTransformer(
    _meta: TransformerMetadata,
    _fn: ValueFn[ImgData | None],
    *,
    alt: Optional[str] = None,
    **kwargs: object,
) -> ImgData | None:

Which defines IT as ImgData | None