text: Title: Caching when no search results found - 2.1.1
  Body: I am using python 3.6, oscar 2.1.1 (django 2.2) with haystack + solr 8, cache is memcached 1.6.9. Didnt fork search app. Caching per site is not turned on. Tried to switch to default cache - disable memcached, didnt help. Settings:
  
  ```
  CACHES = {
      'default': {
          'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
          'LOCATION': '172.11.22.33:11211',
      }
  }
  ```
  modified facet:
  ```
  OSCAR_SEARCH_FACETS = {
      'fields': OrderedDict([
          # ('product_class', {'name': _('Type'), 'field': 'product_class'}),
          ('rating', {'name': _('Rating'), 'field': 'rating'}),
      ]),
      'queries': OrderedDict([
          ('price_range',
           {
               'name': _('Price range'),
               'field': 'price',
               'queries': [
                   # This is a list of (name, query) tuples where the name will
                   # be displayed on the front-end.
                   (_('0 to 20'), u'[0 TO 20]'),
                   (_('20 to 40'), u'[20 TO 40]'),
                   (_('40 to 60'), u'[40 TO 60]'),
                   (_('60+'), u'[60 TO *]'),
               ]
           }),
      ]),
  }
  ```
  
  options:
  
  ```
  'loaders': [
      ('django.template.loaders.cached.Loader', [
          'django.template.loaders.filesystem.Loader',
          'django.template.loaders.app_directories.Loader',
      ]),
  ],
  ```
  
  ### Steps to Reproduce
  1. I have lot of 'cbd' products. When I search for 'cbd', they are correctly found. 
  2. When I search for 'cbw' (/?s=cbw), page with 0 results is rendered. 
  3. After that when I try to search for 'cbd', same template for 'cbw' is rendered again: the new searched text is correctly replaced in url (/?s=cbd), but part of template with results stands the same (Produits correspondant à "cbw": 0 résultat trouvé: seems to be cached. Even when I delete (?s=cbd) from url, page /search/ is still cached with 'Produits correspondant à "cbw": 0 résultat trouvé'.