[3.3 BC] Extensions break on missing load().

[bobdenotter]

After updating from 3.2.7 to 3.3 beta, my Gridview extension broke with this exception:

Can easily be "fixed" by adding this to the class that implements FieldInterface:

    public function load()
    {
        return;
    }

However, this is a BC break, that should be fixed. :-)

[bobdenotter]

Same for missing persist(), by the way.

[bobdenotter]

@rossriley In the case of persist, adding a public function persist() { } also makes the error go away, but it also means that the data input doesn't actually get saved in the DB. :-)

[GwendolenLynch]

Fixed in #6458