add missing default definition

Doctrine reads the schema from the database and receives an emptystring default from postgresql for text columns without a default value. This PR adds the missing definition to the schema so doctrine will no longer consider this a diff.

PR for the apps repo is https://github.com/owncloud/apps/pull/1551

@karlitschek @DeepDiver1975 @bartv2 @kabum @PVince81 please review.

PS: I also suspect issues with oracle ... let us fix PSQL first.