core/cachew: special handling for None in order to preserve cache_dir() path

+ add 'suffix' argument for more straighforward logic

tests/misc.py

@@ -120,3 +120,22 @@ def test_cachew() -> None:
 
     assert list(cf()) == [1, 2, 3]
     assert called == cc
+
+
+def test_cachew_dir_none() -> None:
+    from cachew import settings
+    settings.ENABLE = True # by default it's off in tests (see conftest.py)
+
+    from my.core.cachew import cache_dir
+    from my.core.common import mcachew
+    from my.core.core_config import _reset_config as reset
+    with reset() as cc:
+        cc.cache_dir = None
+        called = 0
+        @mcachew(cache_path=cache_dir() / 'ctest')
+        def cf() -> List[int]:
+            nonlocal called
+            called += 1
+            return [called, called, called]
+        assert list(cf()) == [1, 1, 1]
+        assert list(cf()) == [2, 2, 2]