core: remove possiblity of replacements being null

otherwise every caller would have check for nullity (which was
violated), and such testing is not needed at all, since this field can
be properly initialized every time.

Change-Id: Ice3c566c4e77204c3d6eb0e2b24ec68210cbc8d7
Signed-off-by: Martin Mucha <mmucha@redhat.com>