Correctly check shape not None in Keras add_weight.

When calling Keras add_weight with a np list, as written the `shape or
()` "trick" results in the following exception:
"""ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()"""
This change fixes the problem by using an explicit `if`.

PiperOrigin-RevId: 236407103

tensorflow/python/keras/engine/base_layer.py

@@ -310,7 +310,8 @@ class Layer(trackable.Trackable):
       ValueError: When giving unsupported dtype and no initializer or when
         trainable has been set to True with synchronization set as `ON_READ`.
     """
-    shape = shape or ()
+    if shape is None:
+      shape = ()
     # Validate optional keyword arguments.
     for kwarg in kwargs:
       if kwarg not in ['getter', 'collections', 'experimental_autocast']: