Fix issue caused by None in batch dimension for tf.layers.conv3d (#18027)

* Fix issue caused by None in batch dimension for tf.layers.conv3d

This fix tries to address the issue raised in 15655 where
error returns when the batch dimension for tf.layers.conv3d
is None with "channels_first" format.

This fix cast None to `-1` to address the issue

This fix fixes 15655.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test case for batch dimension = None in tf.layers.conv3d

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Address review feedback

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>