fix error when out is not None test=develop (#24103)

python/paddle/tensor/stat.py

@@ -151,7 +151,7 @@ def std(input, axis=None, keepdim=False, unbiased=True, out=None, name=None):
 
     tmp = var(input, axis=axis, keepdim=keepdim, unbiased=unbiased, name=name)
     tmp = layers.sqrt(tmp)
-    if out:
+    if out is not None:
         layers.assign(input=tmp, output=out)
         return out
     else: