Allow empty sharp image

[strarsis]

For specific applications, like with using overlayWith(...),
it may be necessary or easier to just create an wholly empty image
(maybe with specific dimensions or just 1px x 1px and extend(...) it).

However, using sharp() without any argument
results in non-functional output methods.

One workaround may be using a dummy input image -
but this unnecessarily complicates the application.

[lovell]

Hello, did you see #470, a similar question?

[strarsis]

Yes, this seems to be the same issue.