fix: handle None values in the reference column in the `get_qa_wi…

…th_reference` helper (#4309)

src/phoenix/trace/dsl/helpers.py

@@ -112,7 +112,9 @@ def get_qa_with_reference(
     # Consolidate duplicate rows via concatenation. This can happen if there are multiple
     # retriever spans in the same trace. We simply concatenate all of them (in no particular
     # order) into a single row.
-    ref = df_docs.groupby("context.trace_id")["reference"].apply(lambda x: separator.join(x))
+    ref = df_docs.groupby("context.trace_id")["reference"].apply(
+        lambda x: separator.join(x.dropna())
+    )
     df_ref = pd.DataFrame({"reference": ref})
     df_qa_ref = pd.concat([df_qa, df_ref], axis=1, join="inner").set_index("context.span_id")
     return df_qa_ref