circt: fix build with non clang stdenv

lib.optionalString creates an empty string when the condition is false
And the circt test framework interprets this as disabling all tests
failing the build with `error: filter did not match any tests`

pkgs/development/compilers/circt/default.nix

@@ -49,7 +49,8 @@ stdenv.mkDerivation rec {
   #    https://github.com/NixOS/nixpkgs/issues/214945 discusses this issue.
   #
   # As a temporary fix, we disabled these tests when using clang stdenv
-  LIT_FILTER_OUT = lib.optionalString stdenv.cc.isClang "CIRCT :: Target/ExportSystemC/.*\.mlir";
+  # cannot use lib.optionalString as it creates an empty string, disabling all tests
+  LIT_FILTER_OUT = if stdenv.cc.isClang then "CIRCT :: Target/ExportSystemC/.*\.mlir" else null;
 
   preConfigure = ''
     substituteInPlace test/circt-reduce/test/annotation-remover.mlir --replace "/usr/bin/env" "${coreutils}/bin/env"