Gracefully handle empty user entity references

Problem

When a content type with a user entity reference is given a blank value, a fatal error occurs and the execution stop.

The website encountered an unexpected error. Please try again later.

Drupal\Core\Database\InvalidQueryException: Query condition 'users_field_data.mail IN ()' cannot be empty. in Drupal\Core\Database\Query\Condition->condition() (line 105 of core/lib/Drupal/Core/Database/Query/Condition.php).
Drupal\Core\Database\Query\Select->condition('users_field_data.mail', Array, 'IN') (Line: 61)
Drupal\Core\Entity\Query\Sql\Condition->compile(Object) (Line: 172)
Drupal\Core\Entity\Query\Sql\Query->compile() (Line: 80)
Drupal\Core\Entity\Query\Sql\Query->execute() (Line: 577)
Drupal\Core\Entity\EntityStorageBase->loadByProperties(Array) (Line: 362)
Drupal\excel_importer\Form\ExcelImporterForm->getCorrectValue('reported_event', 'field_name', NULL) (Line: 197)
Drupal\excel_importer\Form\ExcelImporterForm->submitForm(Array, Object)
call_user_func_array(Array, Array) (Line: 114)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 593)
Drupal\Core\Form\FormBuilder->processForm('excel_importer_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 91)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Proposed resolution

The proposed solution here is to skip checking for user entity reference when the value (email) is empty.