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'
             )));
         }
 

lib/Minify/Source.php

@@ -104,7 +104,7 @@ class Minify_Source implements Minify_SourceInterface {
             $this->contentType = $spec['contentType'];
         }
         if (isset($spec['minifier'])) {
-            $this->minifier = $spec['minifier'];
+            $this->setMinifier($spec['minifier']);
         }
         if (isset($spec['minifyOptions'])) {
             $this->minifyOptions = $spec['minifyOptions'];
@@ -130,8 +130,15 @@ class Minify_Source implements Minify_SourceInterface {
     /**
      * {@inheritdoc}
      */
-    public function setMinifier($minifier)
+    public function setMinifier($minifier = null)
     {
+        if ($minifier === '') {
+            error_log(__METHOD__ . " cannot accept empty string. Use 'Minify::nullMinifier' or 'trim'.");
+            $minifier = 'Minify::nullMinifier';
+        }
+        if ($minifier !== null && !is_callable($minifier, true)) {
+            throw new \InvalidArgumentException('minifier must be null or a valid callable');
+        }
         $this->minifier = $minifier;
     }
 

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';
             }
         }
 

lib/Minify/SourceInterface.php

@@ -27,7 +27,7 @@ interface Minify_SourceInterface {
      * @param callable $minifier
      * @return void
      */
-    public function setMinifier($minifier);
+    public function setMinifier($minifier = null);
 
     /**
      * Get options for the minifier