handle 'null' try clause parameters, which can arise in older solc ve…

[kevinclancy]

Notes

In older versions of solc, the generated json ast contains the value null to convey lack of return variables for a try clause. This is translated to None in python. In my recent changes to try/catch IR, I neglected to handle this.

Testing

Try running Slither on the following file with an without these changes:

pragma solidity ^0.6.2;

contract Test {
    function bar() external {}
}

contract Baz {
    function foo(Test a) external {
        try a.bar() {} catch {}
    }
}

Without the changes, an exception is raised. With the changes, Slither runs without errors.

Related Issue

https://github.com/CertiKProject/slither-task/issues/220