Generator strips with zero inputs have their length set to 1 pernamently.
In some cases it is useful to use speed effect on these strips because they
can be animated. This can be done by using their length as is on timeline as
content length.
This is very simplified and temporary solution, as cutting these strips won't
give expected results. Lot of code relies on length of these strips being fixed
to 1, resolving this properly should be done by T59540.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D10026
93c10797dc