Groovy String类型null和empty（）判断

public static void main(String[] args) {
    def a = ""
    def b = null
    if (a?.trim()){  //a为null/""  a?.trim()表达式为false
        println("not empty or null")
    }
    else {
        def c = a?.trim()
        println("empty or null")
    }
}