Ensure traceparent isn't an empty string

Fixes #1369

Signed-off-by: Doug Davis <duglin@gmail.com>

Author: duglin

cloudevents/extensions/distributed-tracing.md

@@ -45,6 +45,7 @@ this extension is being used.
   defined in [section 3.2](https://w3c.github.io/trace-context/#traceparent-header)
 - Constraints
   - REQUIRED
+  - MUST be a non-empty string
 
 ### tracestate