extmod/uasyncio: Handle gather with no awaitables.

This previously resulted in gather() yielding but with no way to be
resumed.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/extmod/uasyncio_gather.py

@@ -53,6 +53,11 @@ async def main():
 
     print("====")
 
+    # Gather with no awaitables
+    print(await asyncio.gather())
+
+    print("====")
+
     # Test return_exceptions, where one task is cancelled and the other finishes normally
     tasks = [asyncio.create_task(task(1)), asyncio.create_task(task(2))]
     tasks[0].cancel()

tests/extmod/uasyncio_gather.py.exp

@@ -9,6 +9,8 @@ Task C: Compute factorial(4)...
 Task C: factorial(4) = 24
 [2, 6, 24]
 ====
+[]
+====
 start 2
 end 2
 [CancelledError(), 2]