Backport PR #55920 on branch 2.1.x (Fix off-by-one in aggregations) (#55942)

Backport PR #55920: Fix off-by-one in aggregations

Co-authored-by: William Ayd <[email protected]>

Author: meeseeksmachine

pandas/core/indexers/objects.py

@@ -102,7 +102,7 @@ def get_window_bounds(
         closed: str | None = None,
         step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
-        if center:
+        if center or self.window_size == 0:
             offset = (self.window_size - 1) // 2
         else:
             offset = 0