fix: perform hook null check before waiting

Last 2 commits introduced fatal null-pointer exception when waiting on
HookRunner object when not set, e.g. waiting for WebhookRunner when no
webhook is defined. Fixed bug and got test_e2e.sh to run successfully.

cmd/git-sync/main.go

@@ -638,14 +638,18 @@ func main() {
 				// Assumes that if hook channels are not nil, they will have at
 				// least one value before getting closed
 				exitCode := 0 // is 0 if all hooks succeed, else is 1
+				if exechookRunner != nil {
 					if err = exechookRunner.WaitForCompletion(); err != nil {
 						log.Error(err, "exechook completed with error")
 						exitCode = 1
 					}
+				}
+				if webhookRunner != nil {
 					if err = webhookRunner.WaitForCompletion(); err != nil {
 						log.Error(err, "webhook completed with error")
 						exitCode = 1
 					}
+				}
 				log.DeleteErrorFile()
 				os.Exit(exitCode)
 			}