Repalce the last activity from 'None' to 'home' when using write_benchmark

[syhwawa]

When I used write_benchmark function to write the trips and legs from population.xml, I found the last activity for every agent’s trip was defined as None which should be home. I have checked all of the None value comes from the last trips.
An easy fix can be replacing the 'None' with 'home' in the function.

trips_input
	seq	purp
0	0	work
1	1	work
2	2	work
3	3	work
4	4	work
5	5	work
6	6	work
7	7	None
8	0	work
9	1	work