Revert change to MiddlewareFactory::prepare()

This patch re-instates the conditional check for non-string or empty
middleware, but with a slight change; it now reads:

```php
if (! is_string($middleware) || $middleware === '') {
    throw Exception\InvalidMiddlewareException::forMiddleware($middleware);
}
```

This satisfies the static analyzer, while simultaneously ensuring the
intended behavior.

Author: weierophinney

src/MiddlewareFactory.php

@@ -76,11 +76,11 @@ public function prepare($middleware) : MiddlewareInterface
             return $this->pipeline(...$middleware);
         }
 
-        if (is_string($middleware)) {
-            return $this->lazy($middleware);
+        if (! is_string($middleware) || $middleware === '') {
+            throw Exception\InvalidMiddlewareException::forMiddleware($middleware);
         }
 
-        throw Exception\InvalidMiddlewareException::forMiddleware($middleware);
+        return $this->lazy($middleware);
     }
 
     /**