SYNTHESIZER/RINGBUFFER: Simplify code, rely on std algorithms

Std algorithms can handle zero length parameter cases, so no need for
an additional branch here.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS/SYNTHESIZER: Add two new ringbuffer tests for zero length parameters

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SYNTHESIZER/RINGBUFFER: Fix too restrictive assert for 0-length parameters

Signed-off-by: Gama <misept.dieseneuf@gmail.com>