Fix merge service error with no source
Fix merge service error with no source

This shouldn't be possible in CE, but is needed so that the code is the
same in CE and EE.

iiet/iiet-git,htve/GitlabForChinese,htve/GitlabForChinese,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,LUMC/gitlabhq,mmkassem/gitlabhq,t-zuehlsdorff/gitlabhq,darkrasid/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,LUMC/gitlabhq,darkrasid/gitlabhq,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,t-zuehlsdorff/gitlabhq,LUMC/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,darkrasid/gitlabhq,stoplightio/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,darkrasid/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,dplarson/gitlabhq,dreampet/gitlab,LUMC/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,htve/GitlabForChinese,iiet/iiet-git,iiet/iiet-git,htve/GitlabForChinese,dreampet/gitlab