Fix web-history-max-items-crash.

Fixes #2849, where pressing 'o' with web-history-max-items set and no
history items would cause a crash as the query result is empty.

qutebrowser/completion/models/histcategory.py

@@ -72,6 +72,10 @@ class HistoryCategory(QSqlQueryModel):
             'ORDER BY last_atime DESC LIMIT :limit)',
         ])).run(limit=max_items).value()
 
+        if not min_atime:
+            # if there are no history items, min_atime may be '' (issue #2849)
+            return ''
+
         return "AND last_atime >= {}".format(min_atime)
 
     def set_pattern(self, pattern):

tests/unit/completion/test_histcategory.py

@@ -126,7 +126,8 @@ def test_set_pattern(pattern, before, after, model_validator, hist):
     ], [
         ('b', 'b', '2017-06-16'),
         ('c', 'c', '2017-05-16'),
-    ])
+    ]),
+    (1, [], []),  # issue 2849 (crash with empty history)
 ])
 def test_sorting(max_items, before, after, model_validator, hist, config_stub):
     """Validate the filtering and sorting results of set_pattern."""