NULL Opcode compatibility

[shargon]

Currently with the last changes of NEO-VM, the null's checks doesn't work as expected and the compiler need to be updated according to this changes

The following code

Is compiled as (ILCode)

But the Ceq opcode is compiled as

So I think that the VM need to be updated in order to accept ' null' too in NUMEQUAL

Fixes: neo-project/neo-devpack-dotnet#115 #214

[erikzhang]

@lightszero Can you check it?

[shargon]

@lightszero you have the unit test and changes for it here neo-project/neo-devpack-dotnet#115 , Please review it :)

[lightszero]

add some fix in neo-project/neo-devpack-dotnet#115 ,