Catch bug if instrument not in external sky map header
This fixes a bug where ext_header['nest']='false' would return True since it is a non-emptry string, by checking the contents of the string instead.
Maintenance will be performed on git.ligo.org, containers.ligo.org, and docs.ligo.org on Tuesday 30 April 2024 starting at approximately 9am PDT. It is expected to take around twenty minutes and there will be several periods of downtime throughout the maintenance. Please address any comments, concerns, or questions to the helpdesk.
This fixes a bug where ext_header['nest']='false' would return True since it is a non-emptry string, by checking the contents of the string instead.