vm: allow pushing empty strings

[ixje]

@luc10921 reported that the following is allowed on the C# side

{ "type": "String", "value": ""  }

After some investigation is seems like the ScriptBuilder simply ignores the value and does not add anything to the stream. It will only push OpCode.PUSHDATA1 and a length 0 to the byte array. This PR fixes this scenario to also accept empty strings

[coveralls]

coverage: 80.744% (-0.007%) from 80.751% when pulling e7c49ef on vm-empty-string into 4ac098c on master.