Change model call default arg values to None instead of bool vals

This fixes an issue with model saving where the concrete function
cannot be matched correctly

merlin/models/tf/models/base.py

@@ -1798,7 +1798,7 @@ def build(self, input_shape=None):
 
         self.built = True
 
-    def call(self, inputs, targets=None, training=False, testing=False, output_context=False):
+    def call(self, inputs, targets=None, training=None, testing=None, output_context=None):
         """
         Method for forward pass of the model.
 
@@ -1820,6 +1820,9 @@ def call(self, inputs, targets=None, training=False, testing=False, output_conte
         Tensor or tuple of Tensor and ModelContext
             Output of the model, and optionally the context
         """
+        training = training or False
+        testing = testing or False
+        output_context = output_context or False
         outputs = inputs
         features = self._prepare_features(inputs, targets=targets)
         if isinstance(features, tuple):