Fix NPE in PayloadDispatcherImpl (#11196)

# What Does This Do

Fixes:

```
java.lang.NullPointerException
  at writer.PayloadDispatcherImpl.addTrace(PayloadDispatcherImpl.java:71)
```

Since `mapper` and `packer` are set on the same codePath, hence they
need both to be nullChecked

# Motivation

# Additional Notes

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Jira ticket: [PROJ-IDENT]

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Author: amarziali

dd-trace-core/src/main/java/datadog/trace/common/writer/PayloadDispatcherImpl.java

@@ -79,14 +79,13 @@ private void selectMapper() {
       if (mapperDiscovery.getMapper() == null) {
         mapperDiscovery.discover();
       }
-
       mapper = mapperDiscovery.getMapper();
-      if (null != mapper && null == packer) {
-        batchTimer =
-            monitoring.newTimer("tracer.trace.buffer.fill.time", "endpoint:" + mapper.endpoint());
-        packer = new MsgPackWriter(new FlushingBuffer(mapper.messageBufferSize(), this));
-        batchTimer.start();
-      }
+    }
+    if (null == packer && null != mapper) {
+      batchTimer =
+          monitoring.newTimer("tracer.trace.buffer.fill.time", "endpoint:" + mapper.endpoint());
+      packer = new MsgPackWriter(new FlushingBuffer(mapper.messageBufferSize(), this));
+      batchTimer.start();
     }
   }