Type error in GuidesAbstractBaseBlock.php

Passing `NULL` to `array_filter` when an array is expected.

Version: 2.1.3 - 2.1.6
Maybe earlier versions as well, have not tested.

```
TypeError: array_filter(): Argument #1 ($array) must be of type array, null given in array_filter() (
line 137 of modules/contrib/localgov_guides/src/Plugin/Block/GuidesAbstractBaseBlock.php).
```

A simple cast-down can solve the issue.