"Choose all" breaks with placeholder

[dburt]

Steps to reproduce:

1. Find a has-many select field showing "Too many objects, use search box above" (or "No objects found")
2. Click "Choose all -->"
3. Click Save

An invalid params hash will include a field with contents like this and cause a validation error:

{
    "product_ids" => [
      "",
      "",
      "Too many objects, use search box above"
    ]
}

Suggested solution is to disable the "Choose all -->" link when a placeholder is present.