Only refresh project name if Rundeck server provides it.

It seems that not all Rundeck servers consistently return the project name
when retrieving a job. Not yet sure in what situations it is or isn't
returned, but since jobs are not allowed to move between projects anyway
it doesn't hurt to just skip refreshing it if the server provides no
value.

builtin/providers/rundeck/resource_job.go

@@ -463,7 +463,14 @@ func jobToResourceData(job *rundeck.JobDetail, d *schema.ResourceData) error {
 	d.Set("id", job.ID)
 	d.Set("name", job.Name)
 	d.Set("group_name", job.GroupName)
+
+	// The project name is not consistently returned in all rundeck versions,
+	// so we'll only update it if it's set. Jobs can't move between projects
+	// anyway, so this is harmless.
+	if job.ProjectName != "" {
 		d.Set("project_name", job.ProjectName)
+	}
+
 	d.Set("description", job.Description)
 	d.Set("log_level", job.LogLevel)
 	d.Set("allow_concurrent_executions", job.AllowConcurrentExecutions)