Minify no longer tries to minify -min.js/.min.js files

2.2 used empty string as a magical value meaning do not minify, but
in the refactoring `Minify::combineMinify` forgot to interpret this
value that way and instead inherited the default compressor for the type.

This eliminates `""` as a magical value, but for BC rewrites it to
`Minify::nullMinifier` in the sources.

Fixes #499

lib/Minify/Controller/MinApp.php

@@ -190,7 +190,7 @@ class Minify_Controller_MinApp extends Minify_Controller_Base {
                 ,'lastModified' => 0
                 // due to caching, filename is unreliable.
                 ,'content' => "/* Minify: at least one missing file. See " . Minify::URL_DEBUG . " */\n"
-                ,'minifier' => ''
+                ,'minifier' => 'Minify::nullMinifier'
             )));
         }