Don't skip poll() when no handles are present. (#8727)

Fixes #7886.

Fixes #7758.

Fixes #6929.

Fixes #3909.

Replaces #8209.

tests/async/t7758.nim

@@ -0,0 +1,17 @@
+discard """
+  file: "t7758.nim"
+  exitcode: 0
+"""
+import asyncdispatch
+
+proc task() {.async.} =
+  await sleepAsync(1000)
+
+when isMainModule:
+  var counter = 0
+  var f = task()
+  while not f.finished:
+    inc(counter)
+    poll()
+
+doAssert counter == 2