Allow empty blocks.

test/empty_block.wren

@@ -0,0 +1,7 @@
+{} // By itself.
+
+// In a statement.
+if (true) {}
+if (false) {} else {}
+
+IO.write("ok") // expect: ok

test/function/empty_block.wren

@@ -0,0 +1,2 @@
+var f = fn {}
+IO.write(f.call) // expect: null

test/method/empty_block.wren

@@ -0,0 +1,5 @@
+class Foo {
+  bar {}
+}
+
+IO.write((new Foo).bar) // expect: null