Fixed dataset_repo migration

[azhavoro]

Motivation and context

Fix migration after #3708

Running migrations:
  Applying dataset_repo.0006_gitdata_format...
2021-10-26 00:37:30,802 DEBG 'runserver' stderr output:
Traceback (most recent call last):
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
psycopg2.errors.NotNullViolation: column "format" contains null values


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/django/manage.py", line 21, in <module>
    execute_from_command_line(sys.argv)
  File "/opt/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/opt/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/opt/venv/lib/python3.8/site-packages/django/core/management/base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/opt/venv/lib/python3.8/site-packages/django/core/management/base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "/opt/venv/lib/python3.8/site-packages/django/core/management/base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "/opt/venv/lib/python3.8/site-packages/django/core/management/commands/migrate.py", line 243, in handle
    post_migrate_state = executor.migrate(
  File "/opt/venv/lib/python3.8/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/opt/venv/lib/python3.8/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/opt/venv/lib/python3.8/site-packages/django/db/migrations/executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/opt/venv/lib/python3.8/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/opt/venv/lib/python3.8/site-packages/django/db/migrations/operations/fields.py", line 104, in database_forwards
    schema_editor.add_field(
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/base/schema.py", line 487, in add_field
    self.execute(sql, params)
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/base/schema.py", line 142, in execute
    cursor.execute(sql, params)
  File "/opt/venv/lib/python3.8/site-packages/cacheops/transaction.py", line 93, in execute
    result = self._no_monkey.execute(self, sql, params)
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/opt/venv/lib/python3.8/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/opt/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
django.db.utils.IntegrityError: column "format" contains null values

How has this been tested?

Manually

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
  - [ ] I have updated the documentation accordingly
  - [ ] I have added tests to cover my changes
  - [ ] I have linked related issues (read github docs)
  - [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

@azhavoro , please add a line into CHANGELOG