fix: honour a falsey key in all_unique

all_unique applied the key with `map(key, iterable) if key else iterable`, so a falsey callable was silently ignored. it now checks `key is not None` like the other key functions, same fix as #1309.

more_itertools/more.py

@@ -4279,7 +4279,7 @@ def all_unique(iterable, key=None):
     seenset_add = seenset.add
     seenlist = []
     seenlist_add = seenlist.append
-    for element in map(key, iterable) if key else iterable:
+    for element in map(key, iterable) if key is not None else iterable:
         try:
             if element in seenset:
                 return False