Make default value '{}' for non-nullable JSON columns (#32354)

Fixes #32353

Author: roji

src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs

@@ -1283,7 +1283,7 @@ private void InitializeJsonColumn(
         columnOperation.ClrType = typeof(string);
         columnOperation.DefaultValue = inline || isNullable
             ? null
-            : GetDefaultValue(columnOperation.ClrType);
+            : "{}";
 
         columnOperation.AddAnnotations(migrationsAnnotations);
     }