Safely access the activation_method attribute

I don't know why this is happening, as we set it in __init__

Author: basepi

elasticapm/base.py

@@ -353,7 +353,7 @@ def get_service_info(self):
  "version": keyword_field(runtime_version),
  },
  }
- if self.activation_method:
+ if getattr(self, "activation_method", None):
  result["agent"]["activation_method"] = self.activation_method
  if self.config.framework_name:
  result["framework"] = {