Updated the migration job to properly support crawling explorations which do not have any history stored. Also, updated the migration job to be more efficient by pushing the migration to the reduction step, ensuring that explorations are only pulled from the data store on a per state basis, rather than for every unique answer.
Updated the migration job to properly support crawling explorations
which do not have any history stored. Also, updated the migration job to
be more efficient by pushing the migration to the reduction step,
ensuring that explorations are only pulled from the data store on a per
state basis, rather than for every unique answer.

souravbadami/oppia,kevinlee12/oppia,AllanYangZhou/oppia,prasanna08/oppia,kevinlee12/oppia,oppia/oppia,kevinlee12/oppia,MAKOSCAFEE/oppia,terrameijar/oppia,terrameijar/oppia,himanshu-dixit/oppia,prasanna08/oppia,oppia/oppia,prasanna08/oppia,prasanna08/oppia,brianrodri/oppia,shaz13/oppia,MAKOSCAFEE/oppia,souravbadami/oppia,AllanYangZhou/oppia,oppia/oppia,oppia/oppia,brianrodri/oppia,brianrodri/oppia,kevinlee12/oppia,souravbadami/oppia,souravbadami/oppia,himanshu-dixit/oppia,himanshu-dixit/oppia,brianrodri/oppia,MAKOSCAFEE/oppia,terrameijar/oppia,shaz13/oppia,shaz13/oppia,himanshu-dixit/oppia,kevinlee12/oppia,brianrodri/oppia,shaz13/oppia,MAKOSCAFEE/oppia,terrameijar/oppia,AllanYangZhou/oppia,prasanna08/oppia,oppia/oppia,souravbadami/oppia,AllanYangZhou/oppia