Fix trace context in event payload (#2205)

Make sure that always a trace context is added to the event payload. But also make sure that if there is already a trace context in the event, do not overwrite it. (This used to be the behavior before tracing without performance. See: https://github.com/getsentry/sentry-python/blob/1.25.1/sentry_sdk/scope.py#L420)

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Co-authored-by: Ivana Kellyerova <ivana.kellyerova@sentry.io>

Author: antonpirker

sentry_sdk/scope.py

@@ -605,11 +605,11 @@ def _drop(cause, ty):
 
         contexts = event.setdefault("contexts", {})
 
-        if has_tracing_enabled(options):
-            if self._span is not None:
+        if contexts.get("trace") is None:
+            if has_tracing_enabled(options) and self._span is not None:
                 contexts["trace"] = self._span.get_trace_context()
-        else:
-            contexts["trace"] = self.get_trace_context()
+            else:
+                contexts["trace"] = self.get_trace_context()
 
         exc_info = hint.get("exc_info")
         if exc_info is not None: