Ajax error when exporting event enrollments

Problem/Motivation

When trying to export event enrolments an AJAX error occurs.

Steps to reproduce

1. Install the social_event_enrollments_export and social_birthday
2. Go to event with enrollments
3. Export the enrollments

Proposed resolution

The issue is that in UserExportPluginBase the method profileGetFieldValue is typed hinted to return a string, but that is not entirely true, because the field can be NULL. So we need to make sure we always return a string.

Remaining tasks

User interface changes

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API changes

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Data model changes

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