fix assigning presentImage in initializer

[hiimtmac]

presetImage was not being set with optional image. Objc version has it

- (instancetype)initWithImage:(UIImage *)image
{
    if (self = [super initWithNibName:nil bundle:nil]) {
        _presetImage = image; // <- HERE!!!!
        _isEmpty = (!image);

        _model = [[UBSignatureDrawingModelAsync alloc] init];
    }

    return self;
}

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