Fix #192 - Add defensive code

[jason-fox]

Proposed changes

Add defensive code to ensure user.extra.visible_attributes is populated.

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[mgaggero]

Hi,
this PR fixes an issue that makes troubles using user creation APIs in the latest versions of Keyrock (experienced with docker container fiware/idm:8.1.0). Any chances of merging this into the next release of code/docker-container?
@jason-fox @apozohue10 @aalonsog

[apozohue10]

Merged! It will be available on the next release