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/minify.php

@@ -55,7 +55,7 @@ if ($paths) {
             $sources[] = new Minify_Source(array(
                 'id' => $path,
                 'content' => "/*** $path not found ***/\n",
-                'minifier' => '',
+                'minifier' => 'Minify::nullMinifier',
             ));
         }
     }