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originally made.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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -25,7 +25,7 @@ public sealed class ModbusDriver
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     // ---- instance fields (grouped at top for auditability) ----