Allow location_id = None for PrevalenceAllLineagesByLocationHandler

[flaneuse]

Currently, this query fails, since no location_id is specified:

https://api.outbreak.info/genomics/prevalence-by-location-all-lineages?other_threshold=0.03&nday_threshold=5&ndays=60

(The query only works if you set location_id, like: https://api.outbreak.info/genomics/prevalence-by-location-all-lineages?location_id=USA_US-CA&other_threshold=0.03&nday_threshold=5&ndays=60)

We would like to have the PrevalenceAllLineagesByLocationHandler to be able to accept arguments where the location_id is not specified. In that case, the aggregated data returned will be for the all data (e.g. Global location), without a location_id filter.

I believe it should be as simple as adding a check if location_id exists, like in CumulativePrevalenceByLocationHandler