Clarify that empty cursors are valid cursors

Author: stephentoub

docs/specification/draft/server/utilities/pagination.mdx

@@ -92,6 +92,9 @@ The following MCP operations support pagination:
 3. Clients **MUST** treat cursors as opaque tokens:
    - Don't make assumptions about cursor format
    - Don't attempt to parse or modify cursors
+   - Don't make any determination based on cursor value other than whether a
+     non-null value was provided (e.g. an empty string is a valid cursor and
+     thus **MUST NOT** be treated as the end of results)
    - Don't persist cursors across sessions
 
 ## Error Handling