NULL $extender breaks PageHeaderBlock

[NikLP]

Fault at line: https://github.com/localgovdrupal/localgov_core/blame/ced43a28fc9f52d5951a66fb282ea191fc1c007c/src/Plugin/Block/PageHeaderBlock.php#L261

Issue: If $extender is NULL, the lede part of the code breaks with:
Error: Call to a member function getLede() on null in Drupal\localgov_core\Plugin\Block\PageHeaderBlock->getLede() (line 265 of modules/contrib/localgov_core/src/Plugin/Block/PageHeaderBlock.php).

Solution: Provide check for null on the extender variable

[andybroomfield]

Fix is to check that the got display extender is extending instance of \Drupal\localgov_core\Plugin\views\display_extender\PageHeaderDisplayExtender and update ViewsPageExtenderTest.php to account for display extender is off.

[andybroomfield]

This occurs if the page header display extender is not switched on, which should be the case in #257 with the update hook, however site builders can switch this off so we should account for it's non presence.

For those waiting for the fix, switching on the Page header display extender manually under /admin/structure/views/settings/advanced resolves this issue.