fix: avoid an extra query on empty spelling suggestions

None was being used as a placeholder to test whether to run
a spelling suggestion query but was also a possible response
when the backend didn’t return a suggestion, which meant 
that calling `spelling_suggestion()` could run a duplicate
query.

Author: acdha

haystack/backends/__init__.py

@@ -18,6 +18,7 @@
 
 VALID_GAPS = ['year', 'month', 'day', 'hour', 'minute', 'second']
 
+SPELLING_SUGGESTION_HAS_NOT_RUN = object()
 
 def log_query(func):
     """
@@ -469,7 +470,7 @@ def __init__(self, using=DEFAULT_ALIAS):
         self._hit_count = None
         self._facet_counts = None
         self._stats = None
-        self._spelling_suggestion = None
+        self._spelling_suggestion = SPELLING_SUGGESTION_HAS_NOT_RUN
         self.spelling_query = None
         self.result_class = SearchResult
         self.stats = {}
@@ -678,7 +679,7 @@ def get_spelling_suggestion(self, preferred_query=None):
         If the query has not been run, this will execute the query and store
         the results.
         """
-        if self._spelling_suggestion is None:
+        if self._spelling_suggestion is SPELLING_SUGGESTION_HAS_NOT_RUN:
             self.run(spelling_query=preferred_query)
 
         return self._spelling_suggestion
@@ -975,7 +976,7 @@ def _reset(self):
         self._results = None
         self._hit_count = None
         self._facet_counts = None
-        self._spelling_suggestion = None
+        self._spelling_suggestion = SPELLING_SUGGESTION_HAS_NOT_RUN
 
     def _clone(self, klass=None, using=None):
         if using is None: