fix build on gcc below v5.2

gcc below 5.2 doesn't support usage of std::unique_ptr as method argument
with a default assignement to nullptr if the type is forward declared.

This patch is a quick fix, that remove the default assignement and
force the value where needed.

Change-Id: If619dceaed9cca9c9f00f3b9b3f92c768f2c1135