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originally made.

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverInstanceActorSkippedTagTests.cs

@@ -248,7 +248,7 @@ public sealed class DriverInstanceActorSkippedTagTests : RuntimeActorTestBase
         }
 
         /// <inheritdoc />
-        public SkippedTagReport GetSkippedTags()
+        public SkippedTagReport? GetSkippedTags()
         {
             if (ThrowOnGetSkippedTags) throw new InvalidOperationException("reporter is broken");
             return _ledger.Snapshot();