fix: Prevent NPE setting null span baggage (#7848)

The baggage collection don’t handle null.
Prevent throwing exception due to bad API call.

Author: PerfectSlayer

dd-trace-core/src/main/java/datadog/trace/core/DDSpanContext.java

@@ -642,6 +642,10 @@ public long getEndToEndStartTime() {
   }
 
   public void setBaggageItem(final String key, final String value) {
+    if (key == null || value == null) {
+      log.debug("Try to set invalid baggage: key = {}, value = {}", key, value);
+      return;
+    }
     if (baggageItems == EMPTY_BAGGAGE) {
       synchronized (this) {
         if (baggageItems == EMPTY_BAGGAGE) {