fix: if a tuple is empty generate obj (we don't have an equivalent …

…to empty tuple in F#)

[converter]

=== changelog ===
```ts
type MyType = []
```

```fs
type MyType =
    obj
```
=== changelog ===

Fix #158

src/Glutinum.Converter/Transform.fs

@@ -348,7 +348,9 @@ let private transformPrimitive (gluePrimitive: GluePrimitive) : FSharpPrimitive
     | GluePrimitive.Never -> FSharpPrimitive.Null
 
 let private transformTupleType (context: TransformContext) (glueTypes: GlueType list) : FSharpType =
-    glueTypes |> List.map (transformType context) |> FSharpType.Tuple
+    match glueTypes with
+    | [] -> FSharpType.Object
+    | _ -> glueTypes |> List.map (transformType context) |> FSharpType.Tuple
 
 module TypeLiteral =
 

tests/specs/references/literalType/emptyTuple.d.ts

@@ -0,0 +1 @@
+type MyType = []

tests/specs/references/literalType/emptyTuple.fsx

@@ -0,0 +1,13 @@
+module rec Glutinum
+
+open Fable.Core
+open Fable.Core.JsInterop
+open System
+
+type MyType =
+    obj
+
+(***)
+#r "nuget: Fable.Core"
+#r "nuget: Glutinum.Types"
+(***)