Odd Multisite Redirect Question

My firm has a website (myfirm.com) but it also runs a multisite installation for clients using a second domain (myfirm-mu.com/site1, myfirm-mu.com/site2, etc.).

Obviously I have a primary blog at myfirm-mu.com, but it is empty and I don’t want anyone to be able to go there. If anyone tries to go to myfirm-mu.com, I want it to forward them to the external website, myfirm.com. But, obviously, the subsites of myfirm-mu.com (/site1, etc.) still need to work.

I’ve seen nuclear options using .htaccess or wp-config, but I’m guessing my solution here might be some kind of plugin.

Can anyone help? Thanks!