uasyncio: wait: Add workaround against heap alloc on empty iteration.

"for a in ():" unconditionally allocates heap so far, per
https://github.com/micropython/micropython/issues/2716 . So, test for
empty result before iterating over it.

uasyncio/uasyncio/__init__.py

@@ -60,6 +60,9 @@ class EpollEventLoop(EventLoop):
         else:
             res = self.poller.poll(delay, 1)
         #log.debug("epoll result: %s", res)
+        # Remove "if res" workaround after
+        # https://github.com/micropython/micropython/issues/2716 fixed.
+        if res:
             for fd, ev in res:
                 cb = self.objmap[fd]
                 if __debug__: