nix: fix postgrest-loadtest-to-markdown on NAs

The python script failed when presenting NAs with:

TypeError: boolean value of NA is ambiguous

Author: steve-chavez

nix/tools/loadtest.nix

@@ -209,6 +209,7 @@ let
         pd.read_json(sys.stdin) \
           .set_index('param') \
           .drop(['branch', 'earliest', 'end', 'latest']) \
+          .fillna("") \
           .convert_dtypes() \
           .to_markdown(sys.stdout, floatfmt='.0f')
       '';