fix: handle invalid index in centerItem and return null for out-of-range subtitle indices

data/subtitle/src/commonMain/kotlin/fr/ajaury/gwenedeg/subtitle/domain/GetCurrentSubtitleIndexUseCase.kt

@@ -4,5 +4,5 @@ class GetCurrentSubtitleIndexUseCase {
     operator fun invoke(
         subtitle: Subtitle,
         progressMs: Long,
-    ): Int = subtitle.lines.indexOfLast { it.startMs <= progressMs }
+    ): Int? = subtitle.lines.indexOfLast { it.startMs <= progressMs }.takeIf { it >= 0 }
 }

data/subtitle/src/commonTest/kotlin/fr/ajaury/gwenedeg/subtitle/GetCurrentSubtitleIndexUseCaseTest.kt

@@ -21,7 +21,7 @@ class GetCurrentSubtitleIndexUseCaseTest {
 
     @Test
     fun returns_minus_one_before_the_first_cue() {
-        assertEquals(-1, getCurrentSubtitleIndex(subtitle, progressMs = -1))
+        assertEquals(null, getCurrentSubtitleIndex(subtitle, progressMs = -1))
     }
 
     @Test
@@ -39,6 +39,6 @@ class GetCurrentSubtitleIndexUseCaseTest {
 
     @Test
     fun returns_minus_one_for_an_empty_subtitle() {
-        assertEquals(-1, getCurrentSubtitleIndex(Subtitle(emptyList()), progressMs = 1_000))
+        assertEquals(null, getCurrentSubtitleIndex(Subtitle(emptyList()), progressMs = 1_000))
     }
 }