Combine empty variable warning messages
When executing a scenario with empty variables, combine them all into a single warning message instead of flooding the log:
06/23/2021 17:15:14,714 INFO [pool-7682-thread-1-main-etl-thread] - WARNING: scenario 'Create User ...' will be executed with empty value for the variable {outcome}
06/23/2021 17:15:14,714 INFO [pool-7682-thread-1-main-etl-thread] - WARNING: scenario 'Create User ...' will be executed with empty value for the variable {outcomedate}
06/23/2021 17:15:14,714 INFO [pool-7682-thread-1-main-etl-thread] - WARNING: scenario 'Create User ...' will be executed with empty value for the variable {member_verification_date}