character varying (CharField) and jsonb (JSONField) fields should be NOT NULL

[jpmckinney]

Similar to https://ocp-software-handbook.readthedocs.io/en/latest/python/django.html#models

• dataset.meta
• dataset_filter.filter_message
• report.data
• data_item.data
• field_level_check.result
• field_level_check_examples.data
• resource_level_check.result
• resource_level_check_examples.data
• dataset_level_check.meta
• time_variance_level_check.meta

In pelican-frontend:

• change null=True to default=dict for FieldLevelCheck.data and ResourceLevelCheckExamples.data.
• remove null=True from CharField
• Remove # noqa: DJ001

Consider doing #112 at same time.