Improve config schema validation

Is the intention here to allow null but not empty string?

Maybe not for now, but I've noticed a lot of the schema validation code here is tricky. There are some libraries we use in other places to automate this (e.g. io-ts), where it's quite simple to define a schema as a TS type and check an object against it at runtime. Would be nice to look at something like that, since it would reduce bug-surface, do more type checks, and enforce our TS types are correct.

E.g. this is how we declaratively define part of the overrides.json schema in Serialized.ts

/**
 * Serialized form of {@see PatchOverride}
 */
const PatchOverrideType = t.type({
  type: t.literal('patch'),
  file: t.string,
  baseFile: t.string,
  baseVersion: t.union([t.undefined, t.string]),
  baseHash: t.string,
  issue: t.union([t.undefined, t.number]),
});

Originally posted by @NickGerleman in https://github.com/microsoft/react-native-windows/pull/7438#discussion_r598198068

Start by reviewing the schema validation code referenced in the issue and the Serialized.ts example for the overrides.json schema. Investigate whether io-ts or a comparable approach can cover the existing validation and TypeScript types. Done means the validation strategy is agreed, the relevant schemas are migrated or clearly scoped, and null versus empty-string behavior is defined.