fix(baggage): oversized header warn only with a header

This commit makes sure that warnings about the baggage header length are
only emitted when the header is actually present, since it does not make
sense to warn about a missing header's length.

Author: Nicolas Marier

Diff for: opentelemetry-api/src/opentelemetry/baggage/propagation/__init__.py

@@ -54,10 +54,11 @@ def extract(
         )
 
         if not header or len(header) > self._MAX_HEADER_LENGTH:
-            _logger.warning(
-                "Baggage header `%s` exceeded the maximum number of bytes per baggage-string",
-                header,
-            )
+            if header:
+                _logger.warning(
+                    "Baggage header `%s` exceeded the maximum number of bytes per baggage-string",
+                    header,
+                )
             return context
 
         baggage_entries = split(_DELIMITER_PATTERN, header)