Address code review feedback - add null checks and clarify comments

Co-authored-by: MPCoreDeveloper <37024522+MPCoreDeveloper@users.noreply.github.com>

Author: Copilot

SharpCoreDB/DataStructures/HashIndex.cs

@@ -29,7 +29,9 @@ public class HashIndex
     public HashIndex(string tableName, string columnName) 
     {
         _columnName = columnName;
-        // Use default comparer for now - SimdHashEqualityComparer has platform-specific issues with boxed integers
+        // Use default comparer: SimdHashEqualityComparer was removed due to issues with
+        // reference equality vs value equality for boxed value types on Linux/.NET 10.
+        // May revisit with proper generic implementation in future.
         _index = new Dictionary<object, List<long>>();
     }
 

SharpCoreDB/DataStructures/Table.cs

@@ -255,7 +255,7 @@ private static int CompareObjects(object? a, object? b)
         }
 
         // Different types - compare as strings
-        return string.Compare(a.ToString(), b.ToString(), StringComparison.Ordinal);
+        return string.Compare(a.ToString() ?? "", b.ToString() ?? "", StringComparison.Ordinal);
     }
 
     /// <summary>