Key Shang

Results 2 issues of Key Shang

Use `createBlock("Mageplaza\BannerSlider\Block\Widget::class")` will result in Exception `Invalid block type: Mageplaza\BannerSlider\Block\Widget::class` and ` Class Mageplaza\BannerSlider\Block\Widget::class does not exist`

When location doesn't set, $value is empty string '', which result in list function exception - Undefined offset.