Improve magic function __get.

Is not necessary another conditionals. When __get is called
is only when the the property is not defined, so
only need return null in order to avoid the
PHP notice of undefined property.

src/Libs/Neuron.php

@@ -38,9 +38,8 @@ class Neuron {
      * @param string $index
      * @return mixed
      */
-    public function __get(string $index) {
-        return (isset($this->{$index}) &&
-                $this->{$index} != '') ? $this->{$index} : null;
+    public function __get(string $index) : mixed {
+        return null;
     }
 }