Fix: Potential empty collection exception

This PR used to fix #2833, but I am no longer able to reproduce the problem in the latest master anyway, so I cannot add a test that this PR would solve at this point.
Still, I think it's safer to have a safeguard. Otherwise, when `result` is `null` and `typeArgs` is empty, it will continue to throw an exception

Source/DafnyCore/AST/Types.cs

@@ -2018,7 +2018,11 @@ public static string PrettyArrowTypeName(string arrow, List<Type> typeArgs, Type
     s += Util.Comma(typeArgs.Take(arity), arg => arg.TypeName(context, parseAble));
     if (domainNeedsParens) { s += ")"; }
     s += " " + arrow + " ";
-    s += (result ?? typeArgs.Last()).TypeName(context, parseAble);
+    if (result != null || typeArgs.Count >= 1) {
+      s += (result ?? typeArgs.Last()).TypeName(context, parseAble);
+    } else {
+      s += "<unable to infer result type>";
+    }
     return s;
   }
 

Source/DafnyLanguageServer.Test/Synchronization/DiagnosticsTest.cs

@@ -51,6 +51,25 @@ decreases y
       await AssertNoDiagnosticsAreComing(CancellationToken);
     }
 
+
+    [TestMethod]
+    public async Task OpeningFunctionWithErrorDoesNotCrash() {
+      var source = @"
+datatype ParseResult<T> = Success(value: T) | Failure
+
+predicate FixMapInner() {
+  forall callback: (string, nat) --> ParseResult<T>, fun: string, u: nat |
+    && fun in underlying.Keys
+    && FixMapSpecInner(underlying.Keys, |input|, callback, u)
+    :: underlying[fun].requires(callback, u)
+}".TrimStart();
+      var documentItem = CreateTestDocument(source);
+      await client.OpenDocumentAndWaitAsync(documentItem, CancellationToken);
+      var diagnostics = await GetLastDiagnostics(documentItem, CancellationToken);
+      Assert.AreEqual(1, diagnostics.Length);
+      await AssertNoDiagnosticsAreComing(CancellationToken);
+    }
+
     [TestMethod]
     public async Task OpeningOpaqueFunctionWorks() {
       var source = @"