Do not create DotLottieImageProvider instance if there's no image files

[hardworker]

LottieFiles dotLottie Exporter 0.1 may produce .lottie files, where an animation contains WebP images not in images folder, but in assets part of .json.

This may lead to an issue while using LottieAnimationLayer's:

public init(
    dotLottie: DotLottieFile?,
    animationId: String? = nil,
    textProvider: AnimationKeypathTextProvider = DefaultTextProvider(),
    fontProvider: AnimationFontProvider = DefaultFontProvider(),
    configuration: LottieConfiguration = .shared,
    logger: LottieLogger = .shared)

It takes dotLottie?.imageProvider which is not nil and cannot decode base64-encoded images inside .json files. However, if it's nil it fallbacks to BundleImageProvider and everything works as expected. So the main idea of my changes is to avoid DotLottieImageProvider creation if there're no images to provide.

[calda]

Can you share an animation file that depends on this behavior to render correctly? That would be a great addition to the snapshot testing suite

[hardworker]

@calda Done!

[calda]

Thanks!