Как определить, что не выбран ни один Radiobutton?

from tkinter import *
root = Tk()
 
 
def show():
    s = f'{var1.get()}, ' \
        f'{var2.get()}'
    lab['text'] = s
 
 
frame = Frame()
frame.pack(side=LEFT)
 
var1 = BooleanVar()
var1.set(0)
c1 = Checkbutton(frame, text="First",
                 variable=var1,
                 onvalue=1, offvalue=0,
                 command=show)
c1.pack(anchor=W, padx=10)
 
var2 = IntVar()
var2.set(-1)
c2 = Checkbutton(frame, text="Second",
                 variable=var2,
                 onvalue=1, offvalue=0,
                 command=show)
c2.pack(anchor=W, padx=10)
 
lab = Label(width=25, height=5, bg="lightblue")
lab.pack(side=RIGHT)
 
root.mainloop()

from tkinter import *
 
root = Tk()
 
 
def show():
    lab['text'] = f'checked: {var1.get()}'
 
 
frame = Frame()
frame.pack(side=LEFT)
 
var1 = IntVar()
var1.set(0)
c1 = Radiobutton(frame, text="First", variable=var1, value=1)
c1.pack(anchor=W, padx=10)
 
c2 = Radiobutton(frame, text="Second", variable=var1, value=2)
c2.pack(anchor=W, padx=10)
lab = Label(width=25, height=5, bg="lightblue")
lab.pack(side=RIGHT)
b1 = Button(frame, text="check", command=show)
b1.pack(anchor=W, padx=10)
 
root.mainloop()