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originally made.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/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISkippedTagReporter.cs

@@ -44,8 +44,15 @@ public interface ISkippedTagReporter
     ///     Must be a pure in-memory read with no I/O — it is called on the health-publish path, is
     ///     deliberately NOT routed through the resilience pipeline, and must never block.
     /// </remarks>
-    /// <returns>The current report; never null. An all-zero report means "nothing skipped".</returns>
-    SkippedTagReport GetSkippedTags();
+    /// <returns>
+    ///     The report from the driver's most recent tag-table build, or <see langword="null"/> when it has
+    ///     not built one yet — a driver that faulted during initialise has <b>no answer</b>, which is not
+    ///     the same as the answer "nothing skipped". Returning a zero report there would state that a
+    ///     driver with a dozen authored tags has none, which is the class of quiet inaccuracy this whole
+    ///     capability exists to remove. (Found on the docker-dev rig, where two faulted drivers each
+    ///     rendered "0 tags" against two authored ones.)
+    /// </returns>
+    SkippedTagReport? GetSkippedTags();
 }
 
 /// <summary>One authored tag a driver could not build into its poll table, and why.</summary>
@@ -71,9 +78,6 @@ public sealed record SkippedTag(string RawPath, string Reason);
 /// </param>
 public sealed record SkippedTagReport(int AuthoredCount, int SkippedCount, IReadOnlyList<SkippedTag> Sample)
 {
-    /// <summary>A driver that has authored nothing and skipped nothing.</summary>
-    public static readonly SkippedTagReport Empty = new(0, 0, []);
-
     /// <summary>Whether any authored tag was skipped — i.e. whether this is worth an operator's attention.</summary>
     public bool HasSkips => SkippedCount > 0;
 
@@ -113,7 +117,9 @@ public sealed class SkippedTagLedger
     public const int MaxSample = 20;
 
     private readonly object _gate = new();
-    private SkippedTagReport _visible = SkippedTagReport.Empty;
+
+    /// <summary>Null until the first <see cref="Commit"/> — "no answer yet", never a zero report.</summary>
+    private SkippedTagReport? _visible;
 
     private int _stagedAuthored;
     private int _stagedSkipped;
@@ -168,8 +174,9 @@ public sealed class SkippedTagLedger
     }
 
     /// <summary>The current published snapshot.</summary>
-    /// <returns>The last committed report, or <see cref="SkippedTagReport.Empty"/> before the first commit.</returns>
-    public SkippedTagReport Snapshot()
+    /// <returns>The last committed report, or <see langword="null"/> before the first commit — a driver
+    /// that has not built a tag table has no answer, and a zero report would be a false one.</returns>
+    public SkippedTagReport? Snapshot()
     {
         lock (_gate) return _visible;
     }