Cannot clear embedVideo field after it was set once

[keep]

Description:
If we fill the embedVideo field of an item once, it is impossible to clear it later.

Steps to reproduce:

• Edit a K2 item and add any value to the embedVideo field.
• Save the item.
• Reopen the item, delete the content of the embedVideo textarea and save.
• The previous video value returns – it cannot be cleared.

Technical details:
K2ModelItem has this logic:
`
if (JRequest::getVar('embedVideo', '', 'post', 'string', JREQUEST_ALLOWRAW)) {
$row->video = JRequest::getVar('embedVideo', '', 'post', 'string', JREQUEST_ALLOWRAW);
}

`
Because of the if condition, an empty string never reaches $row->video, so the field cannot be reset.

Expected behavior:
If the user clears the field, the value in the database should also be cleared.