feat: move nil check to call sites

[SoulPancake]

This addresses

// TODO(rbuckton): Move node != niltest to call sites
@rbuckton Can you please review this
I can undo the stylistic whitespace changes in the internal/ast/utilities.g
but I think they're for the better, LMK what you think

[SoulPancake]

@rbuckton Can you take a look at this?

[SaadiSave]

@SoulPancake Sorry for jumping on here for a slightly tangential issue, but I noticed that nil checks are inconsistent throughout the codebase. For example,

https://github.com/microsoft/typescript-go/blob/main/internal%2Fast%2Fast.go#L15-L31

type Visitor func(*Node) bool

func visit(v Visitor, node *Node) bool {
        if node != nil {
                return v(node)
        }
        return false
}

func visitNodes(v Visitor, nodes []*Node) bool {
        for _, node := range nodes {
                if v(node) {
                        return true
                }
        }
        return false
}

visit checks for nils, but the for loop doesn't check if node is nil. Do you have clear documentation on whether Visitors are supposed to handle nils? Because visit assumes they don't, and visitNodes assumes they do, which are mutually incompatible assumptions.

[SoulPancake]

@SaadiSave Agreed, I will add the nil check in the loop

[SaadiSave]

@SoulPancake is there a lint to enforce nil checks?