Fixes an issue which made it impossible to resume a migration

i.e. when the migration had been completed, and the migration aborted...
when it would be picked up again the next time to do the moving of files
from migration directory to store directory there would be an assumption
that the "migrate" method would have been run before it. This led to an
NPE for an uninitialized field.
Merge pull request #3252 from tinwelint/2.1-resume-migration-fix

[2.1] Fixes an issue which made it impossible to resume a migration