Disallow empty Rendezvous namespace to prevent ambiguous Cookie encoding

[viktorking7]

Description

• Ensures Namespace is never empty:

  • Namespace::new now rejects empty strings with an error.
  • Namespace::from_static panics on empty strings (documented).

• Rationale: an empty namespace serializes identically to None in Cookie wire format, causing semantic collisions (namespace-specific vs. all-namespaces).

• Impact:

  • Hardens Rendezvous Cookie semantics.
  • Potentially breaks callers relying on empty namespaces (now rejected).