[BUGFIX] Image Minify: Skip files without minifier

Early return for all files which have no minifier set,
to avoid an fatal error with the ambigous error message
“Minifier method  cannot be found!”.
For example when webp images exist in the image directory.

The files are added to the error list right away,
to that the task may continue to minify other images and
show a result message (“Minified X of Y files”) at the end.

src/Task/Assets/ImageMinify.php

@@ -389,6 +389,12 @@ protected function minify($files)
                         $minifier = 'svgo';
                         break;
                 }
+
+                // Skip files without available minifier
+                if ($minifier === '') {
+                    $this->results['error'][] = $from;
+                    continue;
+                }
             } else {
                 if (!in_array($this->minifier, $this->minifiers, true)
                     && !is_callable(strtr($this->minifier, '-', '_'))