Fixes an internal server error when no label available

Error occurs when trying to create an issue with no labels at all. In
this case, the POST body parameter will be `"labels":[""]` instead of
the expected `"labels":[]"`.

issues.go

@@ -194,7 +194,9 @@ func (s *IssuesService) CreateIssue(
 	u := fmt.Sprintf("projects/%s/issues", url.QueryEscape(project))
 
 	// This is needed to get a single, comma separated string
-	opt.Labels = []string{strings.Join(opt.Labels, ",")}
+	if len(opt.Labels) > 0 {
+		opt.Labels = []string{strings.Join(opt.Labels, ",")}
+	}
 
 	req, err := s.client.NewRequest("POST", u, opt)
 	if err != nil {