providers/heroku: clean up heroku_app config vars

builtin/providers/heroku/resource_heroku_app.go

@@ -155,17 +155,21 @@ func resourceHerokuAppRead(d *schema.ResourceData, meta interface{}) error {
 		}
 	}
 	for k, v := range app.Vars {
-		if _, ok := care[k]; !ok {
+		if _, ok := care[k]; ok {
 			configVars[k] = v
 		}
 	}
+	var configVarsValue []map[string]string
+	if len(configVars) > 0 {
+		configVarsValue = []map[string]string{configVars}
+	}
 
 	d.Set("name", app.App.Name)
 	d.Set("stack", app.App.Stack.Name)
 	d.Set("region", app.App.Region.Name)
 	d.Set("git_url", app.App.GitURL)
 	d.Set("web_url", app.App.WebURL)
-	d.Set("config_vars", []map[string]string{configVars})
+	d.Set("config_vars", configVarsValue)
 	d.Set("all_config_vars", app.Vars)
 
 	// We know that the hostname on heroku will be the name+herokuapp.com