Facet rebuild fails when date item processor is enabled.

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When using the date item processor to change the granularity of date fields, an error occurs after submitting the search form with one date selected.

TypeError: DateTime::setTimestamp(): Argument #1 ($timestamp) must be of type int, string given in DateTime->setTimestamp() (line 360 of core/lib/Drupal/Component/Datetime/DateTimePlus.php).
call_user_func_array() (Line: 360)
Drupal\Component\Datetime\DateTimePlus->__call() (Line: 263)
Drupal\facets\Plugin\facets\query_type\SearchApiDate->calculateResultFilterAbsolute() (Line: 254)
Drupal\facets\Plugin\facets\query_type\SearchApiDate->calculateResultFilter() (Line: 76)
Drupal\facets\QueryType\QueryTypeRangeBase->build() (Line: 94)
Drupal\facets\Plugin\facets\query_type\SearchApiDate->build() (Line: 141)
Drupal\oe_list_pages\ListBuilder->rebuildFacet() (Line: 71)
Drupal\oe_list_pages\ListBuilder->getDefaultFilterValuesLabel() (Line: 492)

How to reproduce:

1. Enable the test module oe_list_pages_filters_test.
2. Edit a date facet (e.g. the one on the created field).
3. Apply the date item processor.
4. Create a list page, expose the filter.
5. Select one value for it and submit.

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Related OEL ticket: OEL-2418.