spaces: Is `{"via":[]}` considered valid?

**Link to problem area**:
https://spec.matrix.org/v1.14/client-server-api/#mspacechild
https://spec.matrix.org/v1.14/client-server-api/#mspaceparent

**Issue**
The spec currently specifies "When not present or invalid, the child room is not considered to be part of the space.".

This does not specify whether a content of `{"via":[]}` is invalid since it is technically a list of servers but still not routable.

Element-web interprets it as valid while synapse seems to interpret it as invalid. (see element-hq/element-web#29606)

Read the m.space.child and m.space.parent sections of the Matrix Client-Server API specification, then compare the differing interpretations noted for Element Web and Synapse, including element-hq/element-web#29606. Done means the specification explicitly defines whether {"via":[]} is valid or invalid and removes the ambiguity for implementations.

Written by the indexing model from the issue text.