Source sending 0 transport units
You can make it valid depending on your definition of a valid network flow.
Let s and t be the source and sink, respectively. Depending on your problem you may no further restrictions of the network flow conservation of t and s. In most definitions there are even no further restrictions on that, as this would just complicate the definition (without helping much).

If you want to restrict such "superficial" network flows in your example, you may however further state that for each edge (u,v) with a flow (i.e. flow function f(u,v)>0) there must be a path from s to u where each edge on the path has a positive flow (i.e. f(e)>0).

You may further restrict the flow on s not to be negative and the flow on t not beeing positive, i.e. for sum for all u on f(s,u) must be positive and sum for all u on f(u,t) must be positive or 0.

This should then remodel a "more intuitive" network flow comparable with water flows :).

(Note that the restriction: the source must not send out any transport units does not help much to prevent such superficial flows as suggested in the question.)

However, typically one assumes that f(t,u)&lt;=0 for all vertices u, i.e. the sink consumes everything which it retrieves.