Set missing date format type

Problem/Motivation

I investigate why our view with many dates needs so much time. And I saw that
\Drupal\Core\Datetime\DateFormatter::dateFormat via \Drupal\Core\Datetime\DateFormatter::format was called very often.

\Drupal\smart_date\SmartDateTrait use \Drupal\Core\Datetime\DateFormatter::format.

\Drupal\Core\Datetime\DateFormatter::dateFormat is called when it is not type custom.
, but \Drupal\smart_date\SmartDateTrait set this to empty string, which results into unnecessary calls.

See attached screenshots, before and after the fix.

Steps to reproduce

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Proposed resolution

Set missing date time type to custom.

Remaining tasks

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User interface changes

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API changes

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Data model changes

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