handle empty return statements

lox/interpreter.janet

···
                                  nil "nil"
                                  val (string val)))
     [:expr expr] (xprintf (dyn :expr-out @"") "%Q" (evaluate expr))
-    [:return word val] (yield (evaluate val))
+    [:return word val] (yield (when val (evaluate val)))
     [:if cond then else] (do
                            (cond
                              (evaluate cond) (execute then)

test/test_main.janet

···
   52
   102
 `)
+(test-stdout (process `
+  fun p() {
+    print 1;
+    return;
+    print 2;
+  }
+  p();
+`) `
+  1
+`)
 
 (test-error (process "none = nil;") "Undefined variable 'none'.")
 (test-error (process "print none;") "Undefined variable 'none'.")