Optimize null checks when use `IsValid`

[shargon]

I saw a lot of smart contracts that use this code

UInt160 hash = xxxxx;
Assert(hash != null  && hash.IsValid)

this should be optimized to remove the null check because is already done in IsValid

[Jim8y]

from the syntax perspective, if hash is null, it should not be able to call IsValid.

[shargon]

The problem is that we already did a null check in IsValid, so or it's redudant, or we optimize it