Fix AbsoluteFolder.resolveFileById for recursive (#2033)

```kotlin
@Test
    fun testFirst() {
        val f = { a: Int ->
            if (a > 0) a
            else null
        }

        val a = listOf<Int>(-1, -2, -1, -1)
        
        assertEquals(
            a.map{ f(it) }.firstOrNull(),
            null
        )
    }
```

mirai-core/src/commonMain/kotlin/contact/file/AbsoluteFolderImpl.kt

@@ -389,7 +389,7 @@ internal class AbsoluteFolderImpl(
 
         if (!deep) return null
 
-        return folders().map { it.resolveFileById(id, deep) }.firstOrNull()
+        return folders().map { it.resolveFileById(id, deep) }.firstOrNull{ it != null }
     }
 
     override suspend fun resolveFiles(path: String): Flow<AbsoluteFile> {
@@ -495,4 +495,4 @@ internal class AbsoluteFolderImpl(
         result = 31 * result + contentsCount.hashCode()
         return result
     }
-}
+}