Update UniFile

Which has more correct nullability for some methods and case insensitivity for listFiles where possible.

(cherry picked from commit a74a689c9048cc67f4854678fbfefa361631a5e7)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt
#	source-local/src/androidMain/kotlin/tachiyomi/source/local/image/LocalCoverManager.kt

core/src/main/java/tachiyomi/core/util/system/ImageUtil.kt

@@ -337,7 +337,7 @@ object ImageUtil {
                 // Remove pre-existing split if exists (this split shouldn't exist under normal circumstances)
                 tmpDir.findFile(splitImageName)?.delete()
 
-                val splitFile = tmpDir.createFile(splitImageName)
+                val splitFile = tmpDir.createFile(splitImageName)!!
 
                 val region = Rect(0, splitData.topOffset, splitData.splitWidth, splitData.bottomOffset)