[Neo Plugin RPCServer]fix getcontractstate

[Jim8y]

Description

This pr fixes the syntax issue in the getcontractstate method of the rpcserver. The issue is because with the nullable symbol ?, the rpcerror will never be triggered.

Fixes # (issue) #3280

Type of change

• Optimization (the change is only an optimization)
• Style (the change is only a code style for better maintenance or standard purpose)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• TestNotNull_Or

Test Configuration:

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[vncoelho]

LGTM, can you test @mialbu?

[mialbu]

LGTM, can you test @mialbu?

It works, I get Unknown contract as error message now. 💯

{
    "jsonrpc": "2.0",
    "id": 1,
    "error": {
        "code": -102,
        "message": "Unknown contract"
    }
}