VarTag::initialize function wrong parse regex

 According to the [documentation](https://www.phpdoc.org/docs/latest/references/phpdoc/tags/var.html ) `$element_name` is optional

But regex and logic in VarTag::initialize requires it.

Current VarTag variant can't parse first example from documentation
```
class Foo
{
  /** @var string|null Should contain a description */
  protected $description = null;
}
```



