[schloerke]

Fixes #75. An empty {} block in a generator or async function body made the generator loop forever. Examples include generator(function() {}), async(function() {}), and generator(function() { yield(1); {} }).

There were two causes, both in block_states(). An empty function body produced no states, so the main repeat { switch(...) } loop never exited. An empty nested block got spliced into its parent and added nothing, which dropped the final state, so the generator never finished. Both cases now treat {} as NULL, which matches plain R, where function() {} returns NULL.

The tests were written first and fail without the fix. They use a small with_time_limit() helper so that a hang fails the test instead of freezing the suite.