Fix bad extent calculation
Fix bad extent calculation

Nothing stops museum[0] from having a null lat or lon.
Also nulls were being set to the extent if anywhere
in the response. This provides null checks and ensures we
only compare on numbers.

:interviewfail

MuseumStat/imls-museum-data,azavea/imls-museum-data,MuseumStat/imls-museum-data,azavea/imls-museum-data,MuseumStat/imls-museum-data,azavea/imls-museum-data