Support query timeout = 0

In RediSearch query timeout = 0 means unlimited timeout.
In the current implementation, the query timeout is only updated `if timeout` which in case of 0, translates to false.
Since the default timeout of the `query` class is None, replacing the condition with `if self._timeout is not None` will also work for 0.
If the parameter is not a positive integer, redis server will raise an exception.

related issue: redis#2928 redis#2839

redis/commands/search/query.py

@@ -194,7 +194,7 @@ def _get_args_tags(self):
             args += self._ids
         if self._slop >= 0:
             args += ["SLOP", self._slop]
-        if self._timeout:
+        if self._timeout is not None:
             args += ["TIMEOUT", self._timeout]
         if self._in_order:
             args.append("INORDER")