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

min_extras/cli/rewrite-uris.php

@@ -45,7 +45,7 @@ foreach ($paths as $path) {
         $sources[] = new Minify_Source(array(
             'id' => $path,
             'content' => "/*** $path not found ***/\n",
-            'minifier' => '',
+            'minifier' => 'Minify::nullMinifier',
         ));
     }
 }