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/Source/Factory.php

@@ -173,7 +173,7 @@ class Minify_Source_Factory {
                 $spec['minifyOptions']['compress'] = false;
                 // we still want URI rewriting to work for CSS
             } else {
-                $spec['minifier'] = '';
+                $spec['minifier'] = 'Minify::nullMinifier';
             }
         }