Compile lambdas that have an empty lambda set

Closes #3224

This should also fix a number of other issues often observed when a
lambda is never actually called, but we fail to codegen because it has
an empty lambda set.

crates/compiler/test_mono/src/tests.rs

@@ -1682,3 +1682,14 @@ fn choose_u128_layout() {
         "#
     )
 }
+
+#[mono_test]
+fn call_function_in_empty_list() {
+    indoc!(
+        r#"
+        lst : List ({} -> {})
+        lst = []
+        List.map lst \f -> f {}
+        "#
+    )
+}