Allow variables without initializers.

test/variable/global_without_initializer.wren

@@ -0,0 +1,2 @@
+var a
+IO.write(a) // expect: null

test/variable/local_without_initializer.wren

@@ -0,0 +1,4 @@
+{
+  var a
+  IO.write(a) // expect: null
+}