Empty file field causes error on Postgres

how to reproduce:
have drupal 7 with a postgres database, create a form with an optional file upload field and submit the form without a file.

behaviour:

PDOException: SQLSTATE[22P02]: Invalid text representation: 7 ERROR: invalid input syntax for integer: "" LINE 4: WHERE (base.fid IN ('')) ^: SELECT base.fid AS fid, base.uid AS uid, base.filename AS filename, base.uri AS uri, base.filemime AS filemime, base.filesize AS filesize, base.status AS status, base.timestamp AS timestamp, base.uuid AS uuid FROM {file_managed} base WHERE (base.fid IN (:db_condition_placeholder_0)) ; Array ( [:db_condition_placeholder_0] => ) in DrupalDefaultEntityController->load() (Zeile 196 von /home/david/liip/clientis/clientis/includes/entity.inc).

i did not test with mysql, maybe it is more tolerant and this is why the bug does not show.

i can fix this by adjusting webform_webform_submission_presave to skip files with empty values.

  foreach ($node->webform['components'] as $cid => $component) {
    if ($component['type'] == 'file') {
      $has_file_components = TRUE;
      if (!empty($submission->data[$cid]['value'])) {
        // FIX: remove empty values from array
        foreach ($submission->data[$cid]['value'] as $key => $value) {
          if (empty($value)) {
            unset($submission->data[$cid]['value'][$key]);
          } 
        }
        // END FIX
        $new_fids = array_merge($new_fids, $submission->data[$cid]['value']);
      }
    }
  }

workaround: create a custom module with this hook to unset the offending empty entries

function mymodule_webform_submission_presave($node, &$submission) {
  if (isset($submission->file_usage)) {
    foreach ($submission->file_usage as $key => $files) {
      foreach ($files as $index => $fid) {
        if (empty($fid)) {
          unset($submission->file_usage[$key][$index]);
        }
      }
    }
  }
}