Add checks to prevent invalid context error.

[GetToSet]

I found a weird error log regarding invalid CGContext when using this framework.

After some investigation, I found its cause:

CompletionButton is redrawing its background image at traitCollectionDidChange, which is called before layoutSubviews. At early initialization stage, the button may have empty frame with both height and width set to 0, resulting "invalid context error" when drawing background image.

This commit adds extra check to prevent creating context when frame is empty, delaying the creation of background image to layoutSubviews call.

[SvenTiigi]

Hey @GetToSet,

thanks for the investigation and fixing the bug. Great work 👍

[SvenTiigi]

Additionally, I've just tweaked the implementation a little bit (see https://github.com/SvenTiigi/WhatsNewKit/commit/eb588f26dd6410333a61d8e069a24d2c64eaef89)

From

if size.width == 0 && size.height == 0 {
    return nil
}

To

guard size != .zero else {
    return nil
}