Fill engagement activity trend with zeros if null is returned for type.

@rocha @clintonb 

FYI: @shnayder 

Before:
![before](https://cloud.githubusercontent.com/assets/5265058/4449770/b801fe82-481e-11e4-98a3-5dd115073d8f.png)

After:
![after](https://cloud.githubusercontent.com/assets/5265058/4449771/ba0b2bf4-481e-11e4-9450-b6daecadef2b.png)
This can be simplified to: `trend_week[trend_type] = datum[trend_type] or 0`.