fixed linting error and removed redundant test

Author: Akshatt

pandas/core/array_algos/masked_reductions.py

@@ -110,7 +110,7 @@ def max(values: np.ndarray, mask: np.ndarray, *, skipna: bool = True):
 
 
 def mean(values: np.ndarray, mask: np.ndarray, skipna: bool = True):
-    if not values.size:
+    if not values.size or mask.all():
         return libmissing.NA
     _sum = _sumprod(np.sum, values=values, mask=mask, skipna=skipna)
     count = np.count_nonzero(~mask)

pandas/tests/reductions/test_reductions.py

@@ -689,12 +689,6 @@ def test_ops_consistency_on_empty(self, method):
         result = getattr(Series(dtype=float), method)()
         assert pd.isna(result)
 
-        # Empty Mean
-        if method == "mean":
-            s = Series([], dtype=float)
-            result = getattr(s, method)()
-            assert np.isnan(result)
-
         # timedelta64[ns]
         tdser = Series([], dtype="m8[ns]")
         if method == "var":