[DevTask] Explicit null vs undefined

### Current

There are enough discussions on this topic, but as it pertains to Litegraph, the vast majority `null` usage in the library appears to be stylistic, rather than functional.

Strict typing in TypeScript becomes extremely verbose when `null` is used simply as a stand-in for idiomatic `undefined`. Worse still, it often requires runtime guards to guarantee type safety.

### Proposed

Wherever `null` is used for indeterminate purposes, and could simply be replaced with `undefined` (or e.g. just not returning a value), it should be.

Prefer shorthand where appropriate, e.g. optional properties. Adding `| undefined` works for props/params that must be specified, but may not have a value.

Any use of explicit null where it is strict-equality checked can be ignored - it's serving some purpose or another. There are only a handful of instances, anyway.

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