lib.generators: Handle no drvPath in toPretty

Previously, if a derivation without a `drvPath` was handled, an error
would be thrown:

    nix-repl> lib.generators.toPretty {} { type = "derivation"; }
    error: attribute 'drvPath' missing, at /home/infinisil/src/nixpkgs/lib/generators.nix:251:24

With this commit it doesn't anymore:

    nix-repl> lib.generators.toPretty {} { type = "derivation"; }
    "<derivation ???>"

This matches what `nix repl` outputs:

    nix-repl> { type = "derivation"; }
    «derivation ???»

lib/generators.nix

@@ -248,7 +248,7 @@ rec {
          then v.__pretty v.val
       else if v == {} then "{ }"
       else if v ? type && v.type == "derivation" then
-        "<derivation ${v.drvPath}>"
+        "<derivation ${v.drvPath or "???"}>"
       else "{" + introSpace
           + libStr.concatStringsSep introSpace (libAttr.mapAttrsToList
               (name: value: