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.

Test Plan:

• 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