Make null falsey.

test/while/truth.wren

@@ -0,0 +1,26 @@
+// False and null are false.
+while (false) {
+  IO.print("bad")
+  break
+}
+
+while (null) {
+  IO.print("bad")
+  break
+}
+
+// Everything else is true.
+while (true) {
+  IO.print("true") // expect: true
+  break
+}
+
+while (0) {
+  IO.print(0) // expect: 0
+  break
+}
+
+while ("") {
+  IO.print("string") // expect: string
+  break
+}