Fix NullReferenceException in Rabbit. (#7083)

## Summary of changes

We are getting [the
following](https://ddstaging.datadoghq.com/error-tracking?query=source%3Adotnet%20-%2AOutOfMemoryException%2A%20%40tracer_version%3A3.18%2A&refresh_mode=sliding&source=all&sp=%5B%7B%22p%22%3A%7B%22issueId%22%3A%22ddffb7a6-3d7b-11f0-b08f-da7ad0900002%22%7D%2C%22i%22%3A%22error-tracking-issue%22%7D%5D&from_ts=1748947778182&to_ts=1749552578182&live=true)
NullReferenceException.

```
Error : Exception occurred when calling the CallTarget integration continuation.
System.NullReferenceException
at Datadog.Trace.ClrProfiler.AutoInstrumentation.RabbitMQ.PopulateBasicPropertiesHeadersIntegration.OnMethodEnd[TTarget,TReturn](TTarget instance, TReturn returnValue, Exception exception, CallTargetState& state)
at RabbitMQ.Client.Impl.Channel.PopulateBasicPropertiesHeaders[TProperties](TProperties basicProperties, Activity sendActivity, UInt64 publishSequenceNumber)
```

We can reproduce the issue with the following code:

```
BasicProperties property = default; -> NULL
var duckType = property.DuckCast<IBasicProperties>()!; -> duckType = NULL
duckType.Headers ??= new Dictionary<string, object>(); -> NullReferenceException
```

## Reason for change

## Implementation details

## Test coverage

## Other details
<!-- Fixes #{issue} -->

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Author: NachoEchevarria

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/RabbitMQ/PopulateBasicPropertiesHeadersIntegration.cs

@@ -54,7 +54,7 @@ internal static CallTargetState OnMethodBegin<TTarget, TBasicProperties, TActivi
         }
 
         // TReturn is type RabbitMQ.Client.BasicProperties
-        TReturn? basicProperties = default;
+        TReturn? basicProperties;
 
         // PopulateBasicPropertiesHeaders returns null if the supplied IReadOnlyBasicProperties
         // does not have to be modified or if it's a writable instance.
@@ -80,6 +80,10 @@ internal static CallTargetState OnMethodBegin<TTarget, TBasicProperties, TActivi
                 basicProperties = CachedBasicPropertiesHelper<TReturn>.CreateHeaders(state.State!);
             }
         }
+        else
+        {
+            basicProperties = returnValue;
+        }
 
         // duck cast so we can access the Headers property
         var duckType = basicProperties.DuckCast<IBasicProperties>()!;