fix(hosts): a driver that never built a tag table reports NO ANSWER, not zero skips

Found on the docker-dev live gate, not in tests. Two drivers that faulted
during initialise (rtu-gate, opcua1) each rendered "0 tags" on /hosts against
two authored tags apiece, because a ledger that had never committed returned
SkippedTagReport.Empty.

A zero report is a positive claim -- "this driver skipped nothing" -- and
making it on behalf of a driver that never got as far as looking is exactly
the quiet inaccuracy #541 exists to remove. GetSkippedTags() and
SkippedTagLedger.Snapshot() now return null until the first committed build,
so the column renders the same em-dash it already used for a driver that
cannot report at all. Both are 'no answer'; neither is 'nothing skipped'.

Knock-on: a FOCAS driver whose only init threw now reports null rather than a
zero report, which is strictly more honest than the assertion the test
originally made.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Hosts.razor

@@ -234,8 +234,11 @@ else
                                                Healthy, Errors/5 min stays 0, Last error stays empty, and Last
                                                read keeps advancing off the tags that DID map. On the Makino in
                                                #540 that read as a perfectly healthy driver ignoring 20 of its 22
-                                               tags. "—" means the driver cannot report skips, which is distinct
-                                               from reporting none. *@
+                                               tags. "—" means NO ANSWER — either the driver cannot report skips,
+                                               or it faulted before building a tag table — which is distinct from
+                                               the positive claim "this driver skipped nothing". A pre-fix build
+                                               rendered "0 tags" for two faulted drivers on the docker-dev rig
+                                               that had two authored tags apiece. *@
                                             @if (d.SkippedTags is null)
                                             {
                                                 <span class="text-muted">—</span>