[Neo Rpc Methods] fix contract rpc methods parameters · neo-project/neo@c0930c1

Cannot convert null literal to non-nullable reference type.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

Cannot convert null literal to non-nullable reference type.

The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

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