INVITE/RE-INVITE wihout SDP, is that valid?

Short answer is yes, It is totally valid and according RFC, tough you would say it does not make sense, actually can be used in cases where update the Remote Target uri in the dialog is needed.
What the law (RFC 3261) says about that:

Of course, a UAC MAY send a re-INVITE with no session description, in which
case the first reliable non-failure response to the re-INVITE will contain the
offer (in this specification, that is a 2xx response). 

That means if SDP is not present in INVITE, other party must send the SDP in 200 OK as a new offer and UA should send the answer SDP in ACK.
Soemtimes this appraoch is used for "music on hold" and this should be avoided. Its always recommended to send the SDP to hold a call. There are two ways of   acheiving this 
1) Changing the port to zero for the media lines
 2) Or marking the stream as sendonly.
Note that 1) - making the port in media line zero, is more like hack, because is used to disable the stream rather than putting the other end on hold.
So 2) would be the best - putting the call in Hold, modifying the session Parameters and hence SDP is required with "a=inactive" attribute.