Mk `gather_contexts()` support `@acm`s yielding `None`

We were using a `all(<yielded values>)` condition which obviously won't
work if the batched managers yield any non-truthy value. So instead see
the `unwrapped: dict` with the `id(mngrs)` and only unblock once all
values have been filled in to be something that is not that value.

tractor/trionics/_mngrs.py

@@ -70,6 +70,7 @@ async def _enter_and_wait(
     unwrapped: dict[int, T],
     all_entered: trio.Event,
     parent_exit: trio.Event,
+    seed: int,
 
 ) -> None:
     '''
@@ -80,7 +81,10 @@ async def _enter_and_wait(
     async with mngr as value:
         unwrapped[id(mngr)] = value
 
-        if all(unwrapped.values()):
+        if all(
+            val != seed
+            for val in unwrapped.values()
+        ):
             all_entered.set()
 
         await parent_exit.wait()
@@ -91,7 +95,13 @@ async def gather_contexts(
 
     mngrs: Sequence[AsyncContextManager[T]],
 
-) -> AsyncGenerator[tuple[Optional[T], ...], None]:
+) -> AsyncGenerator[
+    tuple[
+        T | None,
+        ...
+    ],
+    None,
+]:
     '''
     Concurrently enter a sequence of async context managers, each in
     a separate ``trio`` task and deliver the unwrapped values in the
@@ -104,7 +114,11 @@ async def gather_contexts(
     entered and exited, and cancellation just works.
 
     '''
-    unwrapped: dict[int, Optional[T]] = {}.fromkeys(id(mngr) for mngr in mngrs)
+    seed: int = id(mngrs)
+    unwrapped: dict[int, T | None] = {}.fromkeys(
+        (id(mngr) for mngr in mngrs),
+        seed,
+    )
 
     all_entered = trio.Event()
     parent_exit = trio.Event()
@@ -116,8 +130,9 @@ async def gather_contexts(
 
     if not mngrs:
         raise ValueError(
-            'input mngrs is empty?\n'
-            'Did try to use inline generator syntax?'
+            '`.trionics.gather_contexts()` input mngrs is empty?\n'
+            'Did try to use inline generator syntax?\n'
+            'Use a non-lazy iterator or sequence type intead!'
         )
 
     async with trio.open_nursery() as n:
@@ -128,6 +143,7 @@ async def gather_contexts(
                 unwrapped,
                 all_entered,
                 parent_exit,
+                seed,
             )
 
         # deliver control once all managers have started up