ruby-modules/bundled-common: do not define gemType for null packages

Without the change `gemType` gets defined via `null` attrset that
exposes unevaluatable attribute:

    nix-repl> ronin.gemType
    error:
       … while evaluating the attribute 'gemType'

         at pkgs/development/ruby-modules/bundled-common/default.nix:122:30:

          121|       inherit ruby bundler gems confFiles envPaths;
          122|       inherit (gems.${pname}) gemType;
             |                              ^
          123|

       … while evaluating an attribute name

       error: value is null while a string was expected

After the change `passthru` does not expose a `gemType` if `pname` is
`null`.

pkgs/development/ruby-modules/bundled-common/default.nix

@@ -117,9 +117,10 @@ let
 
     meta = { platforms = ruby.meta.platforms; } // meta;
 
-    passthru = rec {
-      inherit ruby bundler gems confFiles envPaths;
+    passthru = (lib.optionalAttrs (pname != null) {
       inherit (gems.${pname}) gemType;
+    } // rec {
+      inherit ruby bundler gems confFiles envPaths;
 
       wrappedRuby = stdenv.mkDerivation {
         name = "wrapped-ruby-${pname'}";
@@ -172,7 +173,7 @@ let
             exit 1
           '';
         };
-    };
+    });
   };
 
   basicEnv =