BUG: Avoid undefined behavior ResamplerBase when "Size" isn't specified
BUG: Avoid undefined behavior ResamplerBase when "Size" isn't specified

When the parameter map does not have a "Size" parameters, `ResamplerBase::ReadFromFile()` had undefined behavior, when trying to retrieve the values from an uninitialized local `size` variable. Transformix would then typically produce output like:

> WARNING: The parameter "Size", requested at entry number 1, does not exist at all.
> The default value "14757395258967641292" is used instead.

...

> itk::ExceptionObject
> Location: "ResamplerBase - WriteResultImage()"
> File: Modules\Core\Common\include\itkImportImageContainer.hxx
> Description: Failed to allocate memory for image.
> Error occurred while resampling the image.

Fixed, by zero-initializing the local `size` variable. Accordingly, also adjusted the error message for the case where one or more size values are zero.

Note that with the fix, another error may still occur, but it would be a slightly clearer one:

> ITK ERROR: ImageFileCastWriter: Largest possible region does not fully contain requested paste IO regionPaste IO region: ImageIORegion
>
>  Dimension: 2
>  Index: 0 0
>  Size: 0 0

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix