fix: handle nullable bitmap on Android

[Preeternal]

Description

Fixes an Android compilation failure in DownloadMapImageTask.kt when building with React Native 0.87.1.

CloseableStaticBitmap.underlyingBitmap is nullable, but DownloadMapImageTask calls copy() on it directly.

Reproducer

Build an Android app using:

• React Native 0.87.1
• @rnmapbox/maps 10.3.5
• New Architecture

The build fails during :rnmapbox_maps:compileDebugKotlin with:

DownloadMapImageTask.kt:80:56 Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type 'Bitmap?'.

Verification

After the change, :rnmapbox_maps:compileDebugKotlin succeeds and the Android build proceeds past this task.