fix(Android, FormSheet): Add fallback for undefined corner radius (#3291)

## Description

While working on a different task, I noticed that on Android API level
28 or below, the FormSheet might not display correctly. The issue was
caused by passing a negative corner radius value when it was `undefined`
on the JS side, which `ShapeAppearanceModel` could not handle properly.
This PR adds a fallback to 0 when the value is not provided.

Fixes
software-mansion/react-native-screens-labs#480

## Changes

- Added a fallback for `sheetCornerRadius` prop

## Screenshots / GIFs

### Before
<img width="430" height="848" alt="Screenshot 2025-10-10 at 14 57 47"
src="https://github.com/user-attachments/assets/3dfa849d-d574-4d15-985e-a8e83615e099"
/>

### After
<img width="433" height="844" alt="Screenshot 2025-10-10 at 14 58 10"
src="https://github.com/user-attachments/assets/24fdaaa8-4a1a-4d61-a2e6-c65fd1799f91"
/>

## Test code and steps to reproduce

You can open `GlossyFormSheet` from `TestFormSheet` on Android 28 or
lower; I haven't defined `sheetCornerRadius` there.

## Checklist

- [x] Included code example that can be used to test this change
- [x] Ensured that CI passes

Author: t0maboro

android/src/main/java/com/swmansion/rnscreens/Screen.kt

@@ -34,6 +34,7 @@ import com.swmansion.rnscreens.events.SheetDetentChangedEvent
 import com.swmansion.rnscreens.ext.asScreenStackFragment
 import com.swmansion.rnscreens.ext.parentAsViewGroup
 import com.swmansion.rnscreens.gamma.common.FragmentProviding
+import kotlin.math.max
 
 @SuppressLint("ViewConstructor") // Only we construct this view, it is never inflated.
 class Screen(
@@ -518,7 +519,7 @@ class Screen(
             return
         }
         (background as? MaterialShapeDrawable?)?.let {
-            val resolvedCornerRadius = PixelUtil.toDIPFromPixel(sheetCornerRadius)
+            val resolvedCornerRadius = max(PixelUtil.toDIPFromPixel(sheetCornerRadius), 0f)
             it.shapeAppearanceModel =
                 ShapeAppearanceModel
                     .Builder()

android/src/main/java/com/swmansion/rnscreens/ScreenStackFragment.kt

@@ -39,6 +39,7 @@ import com.swmansion.rnscreens.stack.views.ScreensCoordinatorLayout
 import com.swmansion.rnscreens.transition.ExternalBoundaryValuesEvaluator
 import com.swmansion.rnscreens.utils.DeviceUtils
 import com.swmansion.rnscreens.utils.resolveBackgroundColor
+import kotlin.math.max
 
 sealed class KeyboardState
 
@@ -367,7 +368,7 @@ class ScreenStackFragment :
     }
 
     private fun attachShapeToScreen(screen: Screen) {
-        val cornerSize = PixelUtil.toPixelFromDIP(screen.sheetCornerRadius)
+        val cornerSize = max(PixelUtil.toPixelFromDIP(screen.sheetCornerRadius), 0f)
         val shapeAppearanceModel =
             ShapeAppearanceModel
                 .Builder()