Fixes missing parental ratings & focus loss when changing filters (#1559)

## Description
- Ensures all parent ratings are shown in the filter drop down
- Fixes focus loss after filtering library and there are no results

### Related issues
Fixes #1535 (along w/ #1529)
Fixes #1534

### Testing
Emulator w/ a UK movie library

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/services/FilterOptionCache.kt

@@ -124,9 +124,9 @@ class FilterOptionCache
                         parentId = parentId,
                         userId = userId,
                     ).content.officialRatings
-                    ?.mapNotNull { r ->
-                        val value = ratings[r]
-                        value?.let { FilterValueOption(r, value) }
+                    ?.map { r ->
+                        val value = ratings[r] ?: 0
+                        FilterValueOption(r, value)
                     }?.sortedBy { it.value as Int }
                     .orEmpty()
             }