Fixed Merge Algorithm regarding concurrent file creations

Fixed Merge Algorithm regarding concurrent file creations

When in OURS and THEIRS a new file is created we want a conflict
when the two contents differ. If on two branches the same file
with the same content is created this should not be a conflict.
But: the current merge algorithm is throwing NPEs in this case.
Fix this by choosing an empty RawText as common base if the
base is empty.

Change-Id: I21cb23f852965b82fb82ccd66ec961c7edb3ac3d
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>