Skip fields mappings form if there are not fields created

A mapping form is displayed whenever a teacher applies a preset to a database activity, even if the database does not have any created field yet.

The mappings form has 3 mains functions:

• Define mappings from the current database field to the preset ones
• A checkbox to remove all fields that has no mapping in the preset
• A checkbox (not present yet) to override the activity settings to match the preset ones.

This task should avoid the mappings' form when no field has been created.