refactor: enable no-unnecessary-condition

[sapphi-red]

Description

built on top of #18797

By combining with #18796, we can catch errors like

const { proxy } = config.server
if (proxy) { // this is always `true` should be something wrong
  //
}

Also I found many cases like above, even if we don't enable this rule, I think we should merge the change parts.

[sapphi-red]

Added exhaustiveness check suggested in typescript-eslint/typescript-eslint#10283 (comment)

[sapphi-red]

#18797 (comment)