Allow returns without an explicit value.

test/return/in_function.wren

@@ -1 +1,6 @@
-var a = "ok"; IO.print(a) // expect: ok
+var f = fn {
+  return "ok"
+  IO.print("bad")
+}
+
+IO.print(f.call) // expect: ok

test/return/return_null_if_brace.wren

@@ -0,0 +1,6 @@
+var f = fn {
+  if (true) { return }
+  IO.print("bad")
+}
+
+IO.print(f.call) // expect: null

test/return/return_null_if_newline.wren

@@ -0,0 +1,6 @@
+var f = fn {
+  return
+  IO.print("bad")
+}
+
+IO.print(f.call) // expect: null