Use mail address from user if selected

[pfeuffer]

Proposed changes

If EnvelopeBuilderImpl#fromCurrentUser() is used, this used the mail
address of the current user, if present.

Your checklist for this pull request

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

Contributor:

• PR is well described and the description can be used as a commit message on squash
• New code is covered with unit tests
• Changelog entry file created in gradle/changelog or CHANGELOG.md is updated for plugins

Reviewer:

• The clean code principles are respected (CleanCode)
• All new code/logic is implemented on the right spot / "Should this be done here?"
• UI changes fits into the layout
• The UI views / components are responsive (mobile views)
• Correct translations are available

Checklist for branch merge request (not required for forks)

• Branch is green/blue on Jenkins
• Quality Gate passed on SonarQube