Fix Bazaar migration error

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/three-news-worry.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-bazaar-backend': patch
+---
+
+Handle migration error when old data is present in the database

plugins/bazaar-backend/migrations/20211020073922_add_columns.js

@@ -29,6 +29,7 @@ exports.up = async function up(knex) {
       .comment('Optional end date of the project (ISO 8601 format)');
     table
       .text('responsible')
+      .defaultTo('')
       .notNullable()
       .comment('Contact person of the project');
   });