Fix thread local context not init in Django and add upload command to Makefile (#19)

Makefile

@@ -48,6 +48,9 @@ install: gen
 package: clean gen
 	python3 setup.py sdist bdist_wheel
 
+upload-test: package
+	twine upload --repository-url https://test.pypi.org/legacy/ dist/*
+
 clean:
 	rm -rf browser common language_agent management profile service_mesh_probe
 	rm -rf skywalking_python.egg-info dist build

skywalking/trace/context/__init__.py

@@ -120,6 +120,8 @@ _thread_local.context = None
 
 
 def get_context() -> SpanContext:
+    if not hasattr(_thread_local, 'context'):
+        _thread_local.context = None
     _thread_local.context = _thread_local.context or (SpanContext() if agent.connected() else NoopContext())
 
     return _thread_local.context