It should throw an error when passed empty string as timezone when using moment().tz('')

[shenxiang11]

Moment-timezone version which you use:

Version: latest

Note: many issues are resolved if you just upgrade to the latest version

Issue description:

The function returns undefined, it's not conform to the interface in index.d.js.
I think it should change themoment.Moment to moment.Moment | undefined
or throw an error like we pass a timezone which is not exist?

interface Moment {
        tz(): string | undefined;
        tz(timezone: string, keepLocalTime?: boolean): moment.Moment; // or should be moment.Moment | undefined ?
        zoneAbbr(): string;
        zoneName(): string;
}

[gilmoreorless]

First up, please don't list "latest" in the version field, as it becomes incorrect very quickly. For example, a new version has already been published today. To diagnose issues, we need exact version numbers.

From looking at the code, it seems moment().tz('') is treated exactly the same as moment().tz(), returning string | undefined. This is because of a check of if (zone) which treats the empty string the same as undefined.

I agree that this behaviour doesn't match the type definitions, and isn't quite correct. But fixing it would technically be a breaking change, as code that worked previously could start throwing errors. I'm going to link this to the proposed breaking changes in #1039.