fix: listBackupFiles 跳过 Java 空数组回落 root shell

FUSE 文件系统可能将 EPERM 表现为空数组而非 null，
导致 listBackupFiles 提前返回 [] 从未执行 ls -1 回落。
改为仅当 Java 返回非空结果才提前返回，空数组继续走 root shell。

app/src/main/java/com/example/androidbackupgui/backup/BackupOperation.kt

@@ -460,7 +460,13 @@ object BackupOperation {
      */
     internal suspend fun listBackupFiles(dir: File): List<String>? {
         try {
-            return dir.listFiles()?.map { it.name }
+            val javaFiles = dir.listFiles()
+            if (javaFiles != null) {
+                val names = javaFiles.map { it.name }
+                if (names.isNotEmpty()) return names
+                // Java returned empty — FUSE may report EPERM as empty array
+                // Fall through to root shell ls for definitive answer
+            }
         } catch (_: Exception) { /* fall through */ }
         try {
             val result = RootShell.exec("ls -1 '${dir.absolutePath.shellEscape()}' 2>/dev/null")