RandomGaussianBlur crashes when factor=0 (keras-team#1968)

* RandomGaussianBlur crashes when factor=0

```>>> import keras_cv
Using TensorFlow backend
>>> blurrer = keras_cv.layers.RandomGaussianBlur(3, 0.)
>>> blurrer.get_random_transformation()
(<tf.Tensor: shape=(3, 1, 1, 1), dtype=float32, numpy=
array([[[[nan]]],


       [[[nan]]],


       [[[nan]]]], dtype=float32)>, <tf.Tensor: shape=(1, 3, 1, 1), dtype=float32, numpy=
array([[[[nan]],

        [[nan]],

        [[nan]]]], dtype=float32)>)
```

* Make sure `factor` cannot become too small

* Use keras.backend.epsilon() instead of 0.01

* Update random_gaussian_blur.py

keras_cv/layers/preprocessing/random_gaussian_blur.py

@@ -61,7 +61,9 @@ def __init__(self, kernel_size, factor, **kwargs):
                 )
 
     def get_random_transformation(self, **kwargs):
-        factor = self.factor()
+        # `factor` must not become too small otherwise numerical issues occur.
+        # keras.backend.epsilon() behaves like 0 without causing `nan`s
+        factor = tf.math.maximum(self.factor(), keras.backend.epsilon())
         blur_v = RandomGaussianBlur.get_kernel(factor, self.y)
         blur_h = RandomGaussianBlur.get_kernel(factor, self.x)
         blur_v = tf.reshape(blur_v, [self.y, 1, 1, 1])