Fix nil pointer error when no target url is present

Hot fix for when we post github commit statuses with no target url:

unir_1   | goroutine 52 [running]:
unir_1   | github.com/seemethere/unir/internal.handleStatus(0x2030, 0xc42001606d, 0x9, 0xc4203d58b0, 0xc4203d58f0, 0xc4203d58e0, 0x0, 0xc420224bc0, 0x1, 0x4, ...)
unir_1   | 	/go/src/github.com/seemethere/unir/internal/webhook.go:202 +0x6b
unir_1   | created by github.com/seemethere/unir/internal.(*GithubWebhookHandler).handleGithubWebhook
unir_1   | 	/go/src/github.com/seemethere/unir/internal/webhook.go:52 +0x4b0