ContentEntityDatabaseStorage inserts bogus NULL values for new entities

Here we go.

With this and #2279363: file_managed.uri should not specify a prefixed unique key I can install successfully on PostreSQL again.

Ahem.

The original code first collected $values into a separate array, and then did:

foreach ($values as $field_name => $value) {
  // If we are creating a new entity, we must not populate the record with
  // NULL values otherwise defaults would not be applied.
  if (isset($value) || !$is_new) {
    $record->$field_name = drupal_schema_get_field_value($schema['fields'][$field_name], $value);
    $record->$schema_name = drupal_schema_get_field_value($definition->getSchema()['columns'][$column_name], $value);
  }
}

So basically the same as here except that I killed the second loop and populate $record directly.

I had orginally dropped that code, because I thought it was unneeded as we now apply the defaults from the field definitions, but that a little bit hasty as it now turns out.

Fairly sure this will need to be postponed on #2232477: Fatal when adding new fields with NOT NULL constraints in a base table that contains existing entities :-/. Not doing that yet, though, until we have a green patch there and then can prove that this passes with that patch on top.