Fix os.write.outputStream with perms for Scala 3 (com-lihaoyi#368)

On Scala 3

```scala
os.write.outputStream(os.pwd / "text", perms = os.PermSet(420))

java.lang.ClassCastException: class [Ljava.nio.file.attribute.FileAttribute; cannot be cast to class scala.collection.immutable.Seq ([Ljava.nio.file.attribute.FileAttribute; is in module java.base of loader 'bootstrap'; scala.collection.immutable.Seq is in unnamed module of loader java.net.URLClassLoader @4520ebad)
  os.write$.outputStream(ReadWriteOps.scala:35)
  ammonite.$sess.cmd0$.<clinit>(cmd0.sc:1)
```

Author: kiendang

os/src/ReadWriteOps.scala

@@ -30,8 +30,8 @@ object write {
     checker.value.onWrite(target)
     if (createFolders) makeDir.all(target / RelPath.up, perms)
     if (perms != null && !exists(target)) {
-      val permArray =
-        if (perms == null) Array[FileAttribute[PosixFilePermission]]()
+      val permArray: Array[FileAttribute[_]] =
+        if (perms == null) Array.empty
         else Array(PosixFilePermissions.asFileAttribute(perms.toSet()))
       java.nio.file.Files.createFile(target.toNIO, permArray: _*)
     }

os/test/src/ReadingWritingTests.scala

@@ -124,6 +124,21 @@ object ReadingWritingTests extends TestSuite {
           os.read(wd / "New File.txt") ==> "Hello"
         }
       }
+      test("outputStreamWithPerms") {
+        test - prep { wd =>
+          if (!scala.util.Properties.isWin) {
+            val out = os.write.outputStream(wd / "New File.txt", perms = os.PermSet(420))
+            out.write('H')
+            out.write('e')
+            out.write('l')
+            out.write('l')
+            out.write('o')
+            out.close()
+
+            os.read(wd / "New File.txt") ==> "Hello"
+          }
+        }
+      }
     }
     test("truncate") {
       test - prep { wd =>