Update cdac-build-tool to omit type when it is empty (#101825)

Author: elinor-fung

src/coreclr/tools/cdac-build-tool/DataDescriptorModel.cs

@@ -185,10 +185,6 @@ public DataDescriptorModel Build()
             var globals = new Dictionary<string, GlobalModel>();
             foreach (var (globalName, globalBuilder) in _globals)
             {
-                if (globalBuilder.Type == string.Empty)
-                {
-                    throw new InvalidOperationException($"Type must be set for global {globalName}");
-                }
                 GlobalValue? v = globalBuilder.Value;
                 if (v == null)
                 {

src/coreclr/tools/cdac-build-tool/JsonConverter/FieldModelJsonConverter.cs

@@ -20,7 +20,7 @@ public override DataDescriptorModel.FieldModel Read(ref Utf8JsonReader reader, T
 
     public override void Write(Utf8JsonWriter writer, DataDescriptorModel.FieldModel value, JsonSerializerOptions options)
     {
-        if (value.Type is null)
+        if (string.IsNullOrEmpty(value.Type))
         {
             writer.WriteNumberValue(value.Offset);
         }

src/coreclr/tools/cdac-build-tool/JsonConverter/GlobalModelJsonConverter.cs

@@ -15,7 +15,7 @@ public override DataDescriptorModel.GlobalModel Read(ref Utf8JsonReader reader,
 
     public override void Write(Utf8JsonWriter writer, DataDescriptorModel.GlobalModel value, JsonSerializerOptions options)
     {
-        if (value.Type is null)
+        if (string.IsNullOrEmpty(value.Type))
         {
             // no type: just write 'value' or '[value]'
             JsonSerializer.Serialize(writer, value.Value, options);