Update empty pipeline in set of pipelines with reference audio

[lucianomartin]

The audio_pipeline.c file in the empty pipeline has been split into a _t0 and a _t1 file. The content of the input and output functions have been updated so that they match the same behaviour in the other pipelines.

I built an empty pipeline example by doing:

cmake -B build -G Ninja --toolchain xmos_cmake_toolchain/xs3a.cmake  -DENABLE_ALL_FFVA_PIPELINES=1
cd build 
ninja example_ffva_int_cyberon_empty

and I tested by checking that I can record audio and trigger the intent engine by saying "Hello XMOS".

[shuchitak]

Looks good!