add nil check to z.credential

[goro9]

I had a problem when I wanted to authenticate using OAuth2 token as follows, so I dealt with it.

	ctx := context.Background()
	ts := oauth2.StaticTokenSource(
		&oauth2.Token{AccessToken: os.Getenv("ZENDESK_OAUTH2_TOKEN")},
	)
	tc := oauth2.NewClient(ctx, ts)

	z, _ := zendesk.NewClient(tc)

	ifz.SetSubdomain(os.Getenv("ZENDESK_SUBDOMAIN"))

	ticket, _ := z.GetTicket(ctx, int64(2))

got

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference

Please review.

[nukosuke]

@goro9
LGTM 👍
Thank you for reporting and sending patch for the bug.