Datetime form element cannot validate when using a format without seconds

Problem/Motivation

When using a datetime form element that does not allow to input seconds as follow:

    $form['datetime_without_seconds'] = [
      '#type' => 'datetime',
      '#title' => $this->t('Datetime without seconds'),
      '#default_value' => new DrupalDateTime(),
      '#date_time_element' => 'text',
      '#date_time_format' => 'H:i',
    ];

And submitting the form without changing the value, we get a validation error.
It's caused by the \Drupal\Core\Datetime\Element\Datetime::valueCallback() method that adds ":00" to times that don't have seconds. The associated comment says "Seconds will be omitted in a post in case there's no entry." so I can't understand why it's been done.

Proposed resolution

Only add that seconds when the time format needs it.

Fix the logic in Datetime:valueCallback() for massaging input values so that DrupalDateTime::createFromFormat() does the actual work based on the format specified by the form element.

Remaining tasks

User interface changes

None.

API changes

None.

Data model changes

None.