Fix clip offset due to slate not being taken in to account

[JoseCovaOS]

Here the related issue.

Changelog Description

Issue comes from the following line:

-        slate_on = "slate" in self.context["version"]["data"].get(
-            "families", [])

This data is not stored anymore in self.context["version"]["data"] but in self.context["version"]["attrib"]. Therefore return value was always []. Not entering in this if statement

        # if slate is on then remove the slate frame from beginning
        if slate_on:
            self.media_duration -= 1
            self.handle_start += 1

and causing handles to be -8 +9 instead of -9 +8 in the loaded clip if version had a slate frame (in our tests handles were set to 8 but this happens with any value).

Testing notes:

• Launch Hiero.
• Load a render product that has a slate frame.
• Check handles are correct . It should have a +1 in the start handle instead of at the end (so if handles are 8 start and 8 end, loaded clip should have 9 at the start and 8 at the end).