type: ASTNode types have incorrect optional properties

[yxliang01]

There are some properties of ASTNode types declared as optional via adding ? to ``PROPERTY_NAME (PROPERTY_NAME?`). However, marking optional in this way results in saying such property having value `TYPE | undefined | null` while they are in fact of type `TYPE | null` (or correct me if wrong). e.g.

solidity-parser-antlr/index.d.ts

Line 201 in f0ef770

storageLocation?: string;

CC: @Leeleo3x

[aoli-al]

It is true. Will this introduce any side effect?

[yxliang01]

I see. Side effect as in? @Leeleo3x