vlc: fix eval on aarch64 after #242080

`null`s are not allowed in `env`.

pkgs/applications/video/vlc/default.nix

@@ -202,10 +202,11 @@ stdenv.mkDerivation (finalAttrs: {
   ++ optional (waylandSupport && withQt5) qtwayland;
 
   env = {
-    LIVE555_PREFIX = if stdenv.hostPlatform.isAarch then null else live555;
     # vlc depends on a c11-gcc wrapper script which we don't have so we need to
     # set the path to the compiler
     BUILDCC = "${stdenv.cc}/bin/gcc";
+  } // lib.optionalAttrs (!stdenv.hostPlatform.isAarch) {
+    LIVE555_PREFIX = live555;
   };
 
   patches = [