Undefined `start_year`/`end_year` should mean "no limit", rather than first/last milestone year

If a user leaves the `start_year` for a process empty, I think they would reasonably expect that there should be no lower limit on the installation year for that process.

However, currently, if the user tried to add an existing asset from before the first milestone year, they would get an error like `Process GASDRV does not operate in the year 2010`

This is because we automatically use the first milestone year (in this case 2020) as the `start_year`. To fix this, the user would have to manually set the start year to 2010 or earlier (although it's not really clear from the error message that this is what they have to do).

Ideally, if the user leaves `start_year` empty we should allow existing assets from before the first milestone year, with no lower limit. (I guess it's probably unlikely that users will want to install something in 400BC, so we could reasonably impose a lower limit if we need!)

Practically, I'm not sure how we do this because we store a vec of years for each process rather than a range, and we don't want to have to store _all years_ in the vec, but something to think about.

Linking #845 as well as these may be related