Pure tf.functions can't have None inputs

[faizan-m]

Description

Brief Description of the PR: Using experimental_implements feature now enforces purity on tf.function (all inputs must be Tensors). However, the default value here is None which is not allowed.

Fixes # (issue)

Type of change

• [ x] Bug fix
• [ ] New Tutorial
• [ ] Updated or additional documentation
• [ ] Additional Testing
• [ ] New Activation and the changes conform to the activation contribution guidelines
• [ ] New Callback and the changes conform to the callback contribution guidelines
• [ ] New Image addition and the changes conform to the image op contribution guidelines
• [ ] New Layer and the changes conform to the layer contribution guidelines
• [ ] New Loss and the changes conform to the loss contribution guidelines
• [ ] New Metric and the changes conform to the metric contribution guidelines
• [ ] New Optimizer and the changes conform to the optimizer contribution guidelines
• [ ] New RNN Cell and the changes conform to the rnn contribution guidelines
• [ ] New Seq2seq addition and the changes conform to the seq2seq contribution guidelines
• [ ] New Text addition and the changes conform to the text op contribution guidelines

Checklist:

• [ ] I've properly formatted my code according to the guidelines
  • [ ] By running Black + Flake8
  • [ ] By running pre-commit hooks
• [ ] This PR addresses an already submitted issue for TensorFlow Addons
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] This PR contains modifications to C++ custom-ops

How Has This Been Tested?

If you're adding a bugfix or new feature please describe the tests that you ran to verify your changes: