Double-check the cityMembership endpoint for potential missing data in the database

[hepplerj]

Just noting a couple of things we should look into, @lmullen:

1. I'm noticing that in a few cases the population_1926 field seems empty for some locations. There's a check in the cities-map.js file that, if a null value is found, it'll display that there's missing data to a user (otherwise it breaks the tooltip entirely). But we may want to double-check the database for data should either be there or needs to be added.
2. There seems to be some missing denominations data. For example, if you select "Congregational Methodist Church" there is no data displayed on the map. If it's true there's no data in the database yet for particular denominations, perhaps we should hide cases where there's no/missing data? Or, have some kind of display on the map indicating there isn't data to display.