feat: throw helpful error when appId or privateKey is set to a falsy value

[gr2m]

fixes #184

[github-actions]

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[billyvg]

Just a note that this was a breaking change for us as we were instantiating a client intentionally with auth && privateKey && !installationId, which we use to call apps.getOrgInstallation to get the installationId to create a new client for that org. We now have to have to make sure the installationId property is not in the auth object if it's undefined.