fix(historian): #690 Gateway raw/AtTime Bad rows carry no value (a stored NULL was value 0)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/Mapping/SampleMapper.cs

@@ -23,11 +23,21 @@ internal static class SampleMapper
     /// <summary>Maps a single raw sample to a value snapshot.</summary>
     /// <param name="sample">The gateway raw sample.</param>
     /// <returns>The driver-agnostic snapshot.</returns>
+    /// <remarks>
+    /// #690: a Bad row carries no value. histsdk puts a value on EVERY row (<c>NumericValue = 0</c> on a numeric row,
+    /// <c>StringValue = ""</c> on a string one), so a stored NULL arrives as <c>0</c> / <c>""</c> with a Bad OPC quality
+    /// (live: <c>q=1 opc=0</c>) — that 0 is not a value, and AVEVA stores a Bad-quality sample as NULL, so a Bad row never
+    /// carries a real one. The value is therefore dropped whenever the mapped status is Bad, matching the SqlServer/QuestDB
+    /// backends (an Empty sample is null + Bad) and the aggregate mapper's no-data rows. Uncertain rows keep their value.
+    /// </remarks>
     public static DataValueSnapshot ToSnapshot(HistorianSample sample)
     {
+        var status = ToStatus(sample);
         // proto3 explicit presence: prefer the numeric value, else the string value, else null.
         object? value;
-        if (sample.HasNumericValue)
+        if (StatusBits.IsBad(status))
+            value = null;
+        else if (sample.HasNumericValue)
             value = sample.NumericValue;          // boxes as System.Double
         else if (sample.HasStringValue)
             value = sample.StringValue;
@@ -36,7 +46,7 @@ internal static class SampleMapper
 
         return new DataValueSnapshot(
             Value: value,
-            StatusCode: ToStatus(sample),
+            StatusCode: status,
             SourceTimestampUtc: sample.Timestamp?.ToDateTime(),  // Utc kind
             ServerTimestampUtc: DateTime.UtcNow);
     }