Do not set external context if NULL

This avoids a warning in TFLu

Change-Id: I90f6d6cefbe69c720d3c65c550d7f65e727c48fb
diff --git a/applications/inference_process/src/inference_process.cpp b/applications/inference_process/src/inference_process.cpp
index 4c65005..4ab0353 100644
--- a/applications/inference_process/src/inference_process.cpp
+++ b/applications/inference_process/src/inference_process.cpp
@@ -171,7 +171,9 @@
         model, resolver, tensorArena, tensorArenaSize, &errorReporter, nullptr, &profiler);
 
     // Set external context
-    interpreter.SetMicroExternalContext(job.externalContext);
+    if (job.externalContext != nullptr) {
+        interpreter.SetMicroExternalContext(job.externalContext);
+    }
 
     // Allocate tensors
     TfLiteStatus status = interpreter.AllocateTensors();