Fix empty-targets lookup name var; fail-fast on empty Union/Intersection

Restore handling for lookups whose metadata returns an empty Targets array
(e.g. owningbusinessunit in some Dataverse environments). The refactor in
febde1c dropped the explicit Some [||] -> None guard, causing
getLookupNameVars to render TsType.Union [] as the empty string and emit
broken TypeScript. Type as string when targets are empty.

Also guard TsStringUtil so empty Union/Intersection lists fail at generation
time instead of producing invalid output silently.

Author: Mosh-K

src/CreateTypeScript/CreateWebEntities.fs

@@ -225,15 +225,18 @@ let getLookupNameVars (attrs: XrmAttribute list) =
   attrs
   |> List.filter (fun a -> a.specialType = SpecialType.EntityReference)
   |> List.map (fun a ->
-    let unionType =
-      a.targetEntitySets
-      |> Array.map (fun e -> TsType.Custom $"\"{e.LogicalName}\"")
-      |> Array.toList
-      |> TsType.Union
+    let vType =
+      match a.targetEntitySets with
+      | [||] -> TsType.String
+      | tes ->
+        tes
+        |> Array.map (fun e -> TsType.Custom $"\"{e.LogicalName}\"")
+        |> Array.toList
+        |> TsType.Union
 
     Variable.Create(
       $"\"{valueInfix a.logicalName}@Microsoft.Dynamics.CRM.lookuplogicalname\"",
-      unionType,
+      vType,
       Comment.Attribute(a.displayName, tes = a.targetEntitySets),
       optional = true
     ))

src/TypeScript/TsStringUtil.fs

@@ -30,6 +30,8 @@ and typeToString = function
   | TsType.Generic(n, t)  -> sprintf "%s<%s>" n t
   | TsType.SpecificGeneric(n,ts) 
                           -> sprintf "%s<%s>" n (ts |> Seq.map typeToString |> String.concat ", ")
+  | TsType.Union []       -> failwith "TsType.Union requires at least one type"
+  | TsType.Intersection [] -> failwith "TsType.Intersection requires at least one type"
   | TsType.Union types    -> types |> List.map wrapIfRecursive |> String.concat " | "
   | TsType.Intersection types    -> types |> List.map wrapIfRecursive |> String.concat " & "