The end of a fixed duration frame is stored as a special
`GreasePencilFrame`, notably with a `drawing_index` of
-1.
These were previously called "null" frames (because they
don't point to a drawing). But this name wasn't great.
This commit renames these to the more descriptive
"end" frame. In code, they are `GreasePencilFrame::end()`
and can be checked for with `frame.is_end()`.
All comments and function names referring to "null"
frames have also been updated.
Pull Request: https://projects.blender.org/blender/blender/pulls/121868
e78fbad5fa