I think your modifier is ok. For starters, try:
and remove &operand=`` (although I guess that should work anyway).
And forgive me if this is too basic, but [[+createdby..]] is a placeholder for receiving the output of a snippet, whilst [[*createdby...]] will grab the ’createdby’ field for the current displayed resource. You have both in there, is that intentional?
Unless I misunderstood, you might be able to do away with ’If’ and achieve it this way (not tested though

):
[[*createdby:userinfo=`channelLocation`:notempty=`<a rel="nofollow" href="http://domain.com/[[*createdby:userinfo=`channelLocation`]]">[[*createdby:userinfo=`username`]]</a>`]]
Try both and see which is quicker!