serverForUri should either set ns or be undefined

[isc-klu]

While working on language-server#414, I found that serverForUri can return a ServerForUri with namespace (typed as required string) actually undefined at runtime.

Root cause: AtelierAPI.setConnection computes ns as undefined when neither an explicit namespace nor objectscript.conn.ns is set. That flows through _config/config() and gets asserted non-null where serverForUri destructures it.

Fix: return undefined from serverForUri instead of lying with !.

I am not sure if we want to instead default the namespace to USER.