chore(Android): make ImageLoader onLoaded's result nullable

[kligarski]

Description

Follow-up to #4242. Ensures that onLoaded callback is called even if bitmap was null.

Closes https://github.com/software-mansion/react-native-screens-labs/issues/1589.

Reasoning

E.g. in Stack v5 toolbar menu item implementation when update via view command contains icon, we delay updating the menu item until we load the image. We don't want to miss the onLoaded callback, otherwise we won't update the menu item at all.

Changes

• make onLoaded's result in ImageLoader.loadImage and ImageLoader.loadImageInternal nullable Drawable (Drawable?)
  • no changes were required in existing code that uses the image loader

Before & after - visual documentation

N/A

Test plan

Ensure that images still load in tabs, stack v5.

Checklist

• Ensured that CI passes

[Copilot]

Pull request overview

This PR updates the Android ImageLoader helper to allow onLoaded to receive a nullable Drawable, so callers can still be notified when Fresco produces a null bitmap (e.g., preventing UI updates from being skipped when an icon fails to materialize as a bitmap).

Changes:

• Change ImageLoader.loadImage / loadImageInternal callback type from (Drawable) -> Unit to (Drawable?) -> Unit.
• Call onLoaded(null) when Fresco returns a CloseableStaticBitmap with a null underlying bitmap.

Comments suppressed due to low confidence (1)

android/src/main/java/com/swmansion/rnscreens/gamma/helpers/ImageLoader.kt:30

• loadImage returns early when ImageSource(...).getUri(context) is null, which means the callback is never invoked. Now that onLoaded accepts Drawable?, it should still be called (with null) so callers waiting on the callback can proceed (matching the PR’s goal of not missing onLoaded).

    onLoaded: (Drawable?) -> Unit,
) {
    // Since image loading might happen on a background thread
    // ref. https://frescolib.org/docs/intro-image-pipeline.html
    // We should schedule rendering the result on the UI thread

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[kligarski]

I don't want to introduce such changes now, we can maybe create an issue to look into the image loader in general.

[kligarski]

https://github.com/software-mansion/react-native-screens-labs/issues/1591

[kkafar]

Seems sensible.