fix: should not appear deprecated warning

[ycs77]

Description

I used the Patreon provider for SponsorKit, not adding any GitHub tokens (no SPONSORKIT_LOGIN and SPONSORKIT_GITHUB_LOGIN), but show the warning on build time:

[sponsorkit] env.SPONSORKIT_LOGIN is deprecated, use env.SPONSORKIT_GITHUB_LOGIN instead

Because the getDeprecatedEnv('SPONSORKIT_LOGIN', 'SPONSORKIT_GITHUB_LOGIN') using const value = process.env[name] to get the global variable, the global variable is a string or undefined, if the variable is not found is return the undefined. But it is not equal to null, so the deprecation warning appears when the variable is equal to undefined.

https://github.com/antfu/sponsorkit/blob/0c285ea3dd80fed581ab228b9d021ab83554908a/src/env.ts#L4-L9

So this PR resolved the issue.