Add support for running empty programs

[HCastano]

In entropyxyz/entropy-core#433 the Relayer::register interface was changed so that a program would go from being an Option to a Vec<u8>.

In theory a user can register with an empty program, which would be equivalent to having a None program previously.

However, according to Peg's comment it doesn't look like the execution of an empty program is supported yet. I propose that we allow this but just make it a no-op, essentially allowing a user to jump straight to signature generation.

[jakehemmerle]

Closing since zero-length programs now error; interpretation of error (ie sign empty programs or not) is handled at core level.