Avoid to pass null to MySQLiEngine::escape

Summary:
Several fields in User, Perso and ContentZone are strings,
still undefined when trying to escape them.

As MySQLiEngine::escape accepts strings and not null values,
this change initializes those properties to an empty string.

Fixes T118.
Fixes T119.
Fixes T120.

Test Plan:

• create a new account from an invite
• create a character
• awakens in an unfamiliar place

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T120, T119, T118

Differential Revision: https://code.zed.dereckson.be/D27