Fix an exception in cursor pagination of Conpherence threads
Summary: Ref T13680. Conpherence may pass values with an integer type to this layer of the stack. These are "supposed" to be strings, but just be accepting.
- Wrote 100+ messages to a Conpherence room.
- Clicked "Show Older Messages".
- Before: exception, int passed to "phutil_nonempty_string()".
- After: older messages loaded.
Maniphest Tasks: T13680
Differential Revision: https://secure.phabricator.com/D21824