Fix nullability of settingsStore.ts

`SettingParams` is used as a non-nullable type in several parameters throughout `settingsStore.ts`, however it is called using indexed accessor notation, and tsconfig is not set up to register that this may be nullable.

In several places, optional chaining is used, but not in others. No errors are recorded because TS believes that this value cannot be null.

If true, the optional chaining needs to be removed (see below for inconsistent optional chaining).

If false, fix the undefined prop access.

https://github.com/Comfy-Org/ComfyUI_frontend/blob/1e7aaa42dc56f432754c1cd2a51dc77869eb8d5d/src/stores/settingStore.ts#L28-L35

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4366-Fix-nullability-of-settingsStore-ts-2286d73d365081ae98c0dca34e166efb) by [Unito](https://www.unito.io)

Read src/stores/settingStore.ts at lines 28-35 and inspect the relevant tsconfig settings for indexed access nullability. Use the TypeScript check to determine whether the accessor can be undefined; done means the optional chaining and undefined-property handling are consistent and the nullability diagnostics are resolved.

Written by the indexing model from the issue text.