Michael Strelan
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
+  /**
+   * Checks whether a string is empty.
+   *
+   * This can be used as a callback to functions like array_filter() to avoid
+   * filtering falsy values like 0.
+   *
+   * @param string $text
+   *   The text to check.
+   *
+   * @return bool
+   *   TRUE if the string is empty, FALSE if not.
+   */
+  public static function isEmptyString(string $text): bool {
+    return $text === '';
+  }
+
--- a/core/lib/Drupal/Core/Datetime/Element/Datelist.php
+++ b/core/lib/Drupal/Core/Datetime/Element/Datelist.php
    // \Drupal\Core\Datetime\Element\Datelist::valueCallback().
    unset($input['object']);
    // Filters out empty array values, any valid value would have a string length.
-    $filtered_input = array_filter($input, 'strlen');
+    $filtered_input = array_filter($input, Unicode::isEmptyString(...));