Early exit when the value is an array

core/modules/datetime/src/DateTimeComputed.php

@@ -45,13 +45,19 @@ public function getValue() {
     /** @var \Drupal\Core\Field\FieldItemInterface $item */
     $item = $this->getParent();
     $value = $item->{($this->definition->getSetting('date source'))};
-    $value = is_array($value) ? array_shift($value) : $value;
 
     // A date cannot be created from a NULL value.
     if ($value === NULL) {
       return NULL;
     }
 
+    // When the field is empty, the returned value will be an array with empty
+    // values. Treat it as an empty value. Valid values will return a formatted
+    // string.
+    if (is_array($value)) {
+      return NULL;
+    }
+
     $datetime_type = $item->getFieldDefinition()->getSetting('datetime_type');
     $storage_format = $datetime_type === DateTimeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
     try {