`maxWidth` and `maxHeight` are optional on Android but required on iOS (causing a crash on iOS if not supplied)

[lukehutch]

When calling FlutterImageUtilities.saveAsJpeg, on Android the parameters maxWidth and minWidth don't have to be supplied (they are optional in the Dart API). However, if they are not supplied, then there is a crash in iOS, because of the use of the ! operator:

ImageUtils.swift:

    // save scaled and compressed image to destination file with the original metadata
    let file = try sourceImage!.saveAsJpeg(
        destinationFile: destinationImageFile,
        imageQuality: quality,
        maxSize: Size(width: maxWidth!, height: maxHeight!),            // <------- crashes
        canScaleUp: canScaleUp,
        properties: sourceImageProperties
    )