fix fatal error when the envvar CI_COMMIT_PULL_REQUEST is empty

add test before parse the value as integer

main.go

@@ -324,7 +324,7 @@ func main() {
 		},
 
 		// PullRequest
-		cli.IntFlag{
+		cli.StringFlag{
 			Name:   "pullRequest",
 			Usage:  "pull request number",
 			EnvVar: "CI_COMMIT_PULL_REQUEST",
@@ -407,9 +407,8 @@ func run(c *cli.Context) error {
 			Signed:   c.Bool("yaml.signed"),
 			Verified: c.Bool("yaml.verified"),
 		},
-		Tag:         c.String("tag"),
-		PullRequest: c.Int("pullRequest"),
-		DeployTo:    c.String("deployTo"),
+		Tag:      c.String("tag"),
+		DeployTo: c.String("deployTo"),
 		Config: Config{
 			FromAddress:    fromAddress,
 			FromName:       c.String("from.name"),
@@ -431,5 +430,9 @@ func run(c *cli.Context) error {
 		},
 	}
 
+	if len(c.String("pullRequest")) > 0 {
+		plugin.PullRequest = c.Int("pullRequest")
+	}
+
 	return plugin.Exec()
 }