Fix(dotnet) Add Null safety to doc (#14637)

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## DESCRIBE YOUR PR
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Fix `Value cannot be null. (Parameter 'bytes')` when users copy/paste
this code into their servers.

This happens because on the same doc,  those values can be null
```
properties.Headers.Add("sentry-trace", SentrySdk.GetTraceHeader()?.ToString());
properties.Headers.Add("baggage", SentrySdk.GetBaggage()?.ToString());
```

So we add some null checks before retrieving the trace or baggage and
everyone gets happy!.

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---------

Co-authored-by: Bruno Garcia <[email protected]>

Author: lucas-zimerman

platform-includes/distributed-tracing/custom-instrumentation/dotnet.mdx

@@ -33,12 +33,12 @@ consumer.Received += (model, ea) =>
         var sentryTraceHeader = string.Empty;
         var sentryBaggageHeader = string.Empty;
 
-        if (headers.TryGetValue("sentry-trace", out var traceHeader))
+        if (headers.TryGetValue("sentry-trace", out var traceHeader) && traceHeader is not null)
         {
             sentryTraceHeader = Encoding.UTF8.GetString((byte[])traceHeader);
         }
 
-        if (headers.TryGetValue("baggage", out var baggageHeader))
+        if (headers.TryGetValue("baggage", out var baggageHeader) && baggageHeader is not null)
         {
             sentryBaggageHeader = Encoding.UTF8.GetString((byte[])baggageHeader);
         }