fix: support if without else

If statements without an else should evaluate to nil if the condition is
not valid.  These are mainly used for side effects, but sometimes you
might want a value.

src/env/core.rs

@@ -242,6 +242,11 @@ pub(super) fn core() -> HashMap<String, Node> {
         (
             "if",
             Node::Special(|env, args| {
+                let mut args = args;
+                if args.len() == 2 {
+                    args.push(Node::Nil)
+                }
+
                 arg_count!(3, args.len());
                 let mut args = args.into_iter();
 

src/evaluator.rs

@@ -141,6 +141,8 @@ mod test {
     #[case("(if false true false)", "false")]
     #[case("(if 4 true false)", "true")]
     #[case("(if \"blue\" true false)", "true")]
+    #[case("(if true 3)", "3")]
+    #[case("(if false 3)", "()")]
     // Functions
     #[case("((fn* (a) a) 3)", "3")]
     #[case("((fn* (a) (+ a 2)) 1)", "3")]