[services] Tunnelbroker - Check for an empty string in ConfigManager

Summary:
This diff fixes a bug when the option in the config file provided, but has an empty value the app throws the following error:
what(): basic_string::_M_construct null not valid.
To fix this we will check the option value not to be empty or throw an appropriate error.

Linear task: ENG-1059

Test Plan:
In the config file located at services/tunnelbroker/tunnelbroker.ini change the option value to an empty.
Run service using the command yarn run-tunnelbroker-service and expect the 'Error: config parameter X can not be empty' error.

Reviewers: karol-bisztyga, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4137