super-slicer: handle missing postPatch

When overriding the postPatch attribute from prusa-slicer, handle the
scenario where prusa-slicer doesn't have a postPatch attribute, as well
as the scenario where it does.

pkgs/applications/misc/prusa-slicer/super-slicer.nix

@@ -69,7 +69,7 @@ let
       # - wxScintilla is not used on macOS
       # - Partially applied upstream changes cause a bug when trying to link against a nonexistent libexpat
       postPatch =
-        super.postPatch
+        (super.postPatch or "")
         + ''
           substituteInPlace src/CMakeLists.txt \
             --replace "scintilla" "" \