MB-12053: Set Breakpad libraries variable to empty string if not found

Commit 969ead failed due to cmake setting BREAKPAD_LIBRARIES to
'NOTFOUND' if Breakpad was not present. When the libraries were
linked to memcached the build would fail. This is fixed by
explicitly setting BREAKPAD_LIBRARIES to empty string if they are
not found.

Change-Id: Ic46dc6a9c69785eb13afcc2cd1f142d0924ea6c7
Reviewed-on: http://review.couchbase.org/44475
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>