Fix broken type in getPreviewClassName

[qeleb]

There is a mismatch in the easy-email types that leads to an annoying bug. This makes no difference (as idx is falsy checked before use), but will fix the type issue.

https://github.com/zalify/easy-email/blob/b9b6a5d82fa3718847a8c4ec2abec9a9365d1e93/packages/easy-email-core/src/utils/getPreviewClassName.ts#L4

Change to include undefined like this:

export function getPreviewClassName(idx: string | null | undefined, type: string) {

Can someone commit this exact change?