title: When I access the page first time, .is_valid return false
body: <p>When I access the index page the first time, .is_valid return false. How can I make it not to show False message? Why it shows False when I access the page first time? Is there any way to avoid the False message?</p>
  
  <p>urls.py is below</p>
  
  <pre><code>
  from django.urls import path
  from . import views
  
  urlpatterns = (
      path('', views.index, name='index'),
  )
  </code></pre>
  
  <p>views.py is below</p>
  
  <pre><code>
  
  from django.shortcuts import render
  from . import forms
  
  
  def index(request):
      form = forms.Page1(request.GET or None)
      if form.is_valid():
          message = 'True'
      else:
          message = 'False'
      content = {
          'title': 'Webapp',
          'form': form,
          'message': message,
      }
      return render(request, 'forms.html', content)
  </code></pre>
  
  <p>forms.py is below</p>
  
  <pre><code>
  from django import forms
  
  EMPTY_CHOICES = (
      ('', '-'*10),
  )
  
  GENDER_CHOICES = (
      ('man', '男'),
      ('woman', '女')
  )
  
  
  class Page1(forms.Form):
      user_sir_name = forms.CharField(
          label='姓:',
          max_length=20,
          required=True,
          widget=forms.TextInput(),
      )
      user_given_name = forms.CharField(
          label='名:',
          max_length=20,
          required=True,
          widget=forms.TextInput(),
      )
      user_sex = forms.ChoiceField(
          label='性別:',
          widget=forms.Select,
          choices=EMPTY_CHOICES + GENDER_CHOICES,
          required=True,
      )
  </code></pre>