Accept parameter value `null` in neo-invoke.json interactive mode

[mialbu]

When trying to invoke the transfer method of the NeoToken, it is not intuitive what to do when one does not want to pass any data (i.e., the 4th parameter). I tried with simply null but that disregards that parameter and tries to invoke a method transfer with only 3 parameters (which doesn't exist and thus fails).

Suggestion: Allow passing a value in interactive mode which would translate to the opcode PUSHNULL in the invoke script. For example, simply null like mentioned above.

[mialbu]

New observation: If you write null in the interactive mode, nothing is added to the actual .json file. If I add null manually in the JSON editor mode, it converts to the PUSHNULL opcode in the invoke script as expected.