Make null falsey.

test/if/truth.wren

@@ -0,0 +1,32 @@
+class Iter {
+  new(value) { _value = value }
+  iterate(iterator) { return _value }
+  iteratorValue(iterator) { return "value" }
+}
+
+// False and null are false.
+for (n in new Iter(false)) {
+  IO.print("bad")
+  break
+}
+
+for (n in new Iter(null)) {
+  IO.print("bad")
+  break
+}
+
+// Everything else is true.
+for (n in new Iter(true)) {
+  IO.print("true") // expect: true
+  break
+}
+
+for (n in new Iter(0)) {
+  IO.print(0) // expect: 0
+  break
+}
+
+for (n in new Iter("")) {
+  IO.print("string") // expect: string
+  break
+}