Exception when merging ticket branch in UI

[flaix]

When merging a ticket branch in the web UI, while being logged in as a user without an email address set, an Internal Error is thrown.
The merge goes through, but the web UI returns an error page. This does not happen when the logged in user has an email address set.

java.lang.NullPointerException
	at com.gitblit.git.GitblitReceivePack.updateIncrementalPushTags(GitblitReceivePack.java:448)
	at com.gitblit.git.PatchsetReceivePack.merge(PatchsetReceivePack.java:1313)
	at com.gitblit.wicket.pages.TicketPage$15$1.run(TicketPage.java:1451)
	at com.gitblit.tickets.ITicketService.exec(ITicketService.java:1426)
	at com.gitblit.wicket.pages.TicketPage$15.onClick(TicketPage.java:1443)