Empty `append-bootargs` is gives an error.

[mpolitzer]

Context

What is the bug that you are experiencing?

An empty boot-args like this: --append-bootargs="" gives the following error.

lua5.4: .../dev/cartesi/sdk-bugfix/emulator/src/cartesi-machine.lua:1203: unrecognized option --append-bootargs=
stack traceback:
        [C]: in function 'error'
        .../dev/cartesi/sdk-bugfix/emulator/src/cartesi-machine.lua:1203: in field 'integer index'
        .../dev/cartesi/sdk-bugfix/emulator/src/cartesi-machine.lua:1212: in main chunk
        [C]: in ?

Steps to reproduce

Latest emulator, commit: 9660bd0 (when the issue was created)
Run: cartesi-machine --append-bootargs="".