fix: avoid sending empty Baggage header (#1507)

According to W3C Working Draft spec, the Baggage header must contain at least one value, an empty value is invalid.

Co-authored-by: Neel Shah <[email protected]>

Author: intgr

sentry_sdk/tracing.py

@@ -308,7 +308,9 @@ def iter_headers(self):
             yield "tracestate", tracestate
 
         if self.containing_transaction and self.containing_transaction._baggage:
-            yield "baggage", self.containing_transaction._baggage.serialize()
+            baggage = self.containing_transaction._baggage.serialize()
+            if baggage:
+                yield "baggage", baggage
 
     @classmethod
     def from_traceparent(