_decoder.get_next() may return None

[radarhere]

Suggestions for python-pillow#7311

Three changes

• you've removed the only instance of self._reset(reset=False). If "reset" is always true, then def _reset(self, reset=True) can just become def _reset(self)
• self._decoder.get_next() may return None. See https://github.com/python-pillow/Pillow/blob/15e52290303008c0888dc17aab069edb5cff3a25/src/PIL/WebPImagePlugin.py#L128-L132. So I've added a change to handle that
• I've made a change to the tests so that they fail without your change and pass with them. This will help prevent your change from being undone in the future.