Similar to bug 94586 that discusses inserting shapes with a non-zero border line width, trendlines should also be inserted with a non-zero value, because a zero value should likely mean that it is invisible rather than it currently being visible.
This is going to be a loosing battle. There are so many documents out there that you can't change the handling of zero width trendlines.
It is also not a chart sidebar issue. All trendlines are by default created with a zero width line.
The intent isnt about changing existing documents which value already inserted it with a zero value, its about making sure that new docs are created with a non-zero value.
Here are the default widths used in other apps: Excel 2010: 0.75pt FreeOffice: 0.03"