fix the condition when activation name is 0

mindspore/nn/layer/activation.py

@@ -549,9 +549,9 @@ def get_activation(name):
     Examples:
         >>> sigmoid = nn.get_activation('sigmoid')
     """
-    if not name:
+    if name is None:
         return None
 
     if name not in _activation:
-        raise KeyError("Unknown activation type")
+        raise KeyError(f"Unknown activation type '{name}'")
     return _activation[name]()

tests/ut/python/nn/test_dense.py

@@ -76,7 +76,7 @@ class Net(nn.Cell):
                  weight='normal',
                  bias='zeros',
                  has_bias=True,
-                 activation=''):
+                 activation=None):
         super(Net, self).__init__()
         self.dense = nn.Dense(input_channels,
                               output_channels,

tests/ut/python/pynative_mode/nn/test_activation.py

@@ -46,10 +46,6 @@ def test_activation_param():
     assert isinstance(output_np[0][0][0][0], (np.float32, np.float64))
 
 
-def test_activation_empty():
-    assert nn.get_activation('') is None
-
-
 # test softmax
 def test_softmax_axis():
     layer = nn.Softmax(1)