Tests do not exist for ensuring i2s_frame_master/slave operate with null I/O ports

[ACascarino]

Issues:

• Input or output port specifications are nullable for the i2s_frame_master/slave.
• No current test framework ensures that if these parameters are nulled, the library continues to function as expected.
• The current 4b port implementation will fail if either of these parameters are nulled, despite passing all existing tests. (see Current 4b port implementation fails with runtime error if I/O ports are nulled #114)

[ACascarino]

To complete in tandem with #104