Let value_to_float handle None values

convert.py

@@ -170,9 +170,7 @@ def convert_sdmx_element(element, dataset_json, dataset_context, dsd_infos, seri
 
         timings["observations_labels"] += time.time() - t0
 
-        obs_value = element.attrib.get("OBS_VALUE")
-        if obs_value is not None:
-            obs_value = observations.value_to_float(obs_value)
+        obs_value = observations.value_to_float(element.attrib.get("OBS_VALUE"))
         dataset_context["current_series_observations"].append([
             element.attrib["TIME_PERIOD"],  # SDMX periods are already normalized.
             obs_value,