ghc-pkg: Restore old behavior in colored version; fixes 6119

The behavior is changed to this:
```
ghc-pkg list blahblah
/home/sibi/ghc/inplace/lib/package.conf.d
    (no packages)
```
instead of:
```
ghc-pkg list blahblah
/home/sibi/ghc/inplace/lib/package.conf.d
```

Reviewers: austin, thomie, bgamari

Reviewed By: thomie, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1575

GHC Trac Issues: #6119

utils/ghc-pkg/Main.hs

@@ -1271,10 +1271,13 @@ listPackages verbosity my_flags mPackageName mModuleName = do
     mapM_ show_normal stack
 #else
     let
-       show_colour withF db =
-           mconcat $ map (<#> termText "\n") $
-               (termText (location db) :
-                  map (termText "   " <#>) (map pp_pkg (packages db)))
+       show_colour withF db@PackageDB{ packages = pkg_confs } =
+           if null pkg_confs
+           then termText (location db) <#> termText "\n    (no packages)\n"
+           else
+               mconcat $ map (<#> termText "\n") $
+                           (termText (location db) :
+                                     map (termText "   " <#>) (map pp_pkg pkg_confs))
           where
                    pp_pkg p
                      | installedComponentId p `elem` broken = withF Red  doc