Replace nan with empty string in distribution response (#55)

* replace nan with empty string

* remove print statement

src/hutch_bunny/core/query_solvers.py

@@ -381,7 +381,8 @@ def solve_query(self) -> Tuple[str, int]:
         )
         for _, row in concepts_df.iterrows():
             df.loc[df["OMOP"] == row["concept_id"], "OMOP_DESCR"] = row["concept_name"]
-
+        # replace NaN values with empty string
+        df = df.fillna('')
         # Convert df to tab separated string
         results = list(["\t".join(df.columns)])
         for _, row in df.iterrows():
@@ -564,6 +565,7 @@ def solve_distribution(
         res_b64_bytes = base64.b64encode(res.encode("utf-8"))  # bytes
         size = len(res_b64_bytes) / 1000  # length of file data in KB
         res_b64 = res_b64_bytes.decode("utf-8")  # convert back to string, now base64
+
         result_file = File(
             data=res_b64,
             description="Result of code.distribution anaylsis",