rlog: Skip rlog entry with missing header in RlogConv.parse_rlog

CVS rlog for a given module sent by server is a concatenation of
rlog entries. Each entry has a header containing the path to a
RCS file plus other info.

It exist cases where a rlog entry header is empty which makes the
rlog parsing fail.

So instead of stopping rlog parsing by raising an exception, prefer
to skip that entry and process the next one.

Closes T4629