Add script test to prove that OP_0 evaluates as the empty vector, rather than [0]. Also adds debug output in case an script_invalid.json test fails.
Remove incorrect execution of OP_0. That code was never reached, because OP_0 is not an opcode in terms of chunk.isOpCode()). However, it lead to believe that OP_0 pushes the vector [0], rather than correctly the empty vector to the stack.

Because the code was never executed, this bug could never trigger a test. Afaict, script.cpp does not have the corresponding case in its switch block.
This changes the ignored stack element CHECKMULTISIG bug workaround from the [0] vector to just the empty vector, by using the opcode OP_0 rather than 0x4c 0x00.

Afaict, this is also the workaround behaviour of script.cpp, which uses OP_0, pushing an empty vector. This saves one byte in multisig input scripts.
Remove OP_0 from Script.encodeToOpN() and .decodeFromOpN(). It was incorrectly assumed that OP_0 pushes a [0] vector to the stack. In fact, it pushes the empty vector.