The historical-pricing/v1/views/interday-summaries/ and /intraday-summaries APIs allow to selectively request data for specific fields only (called "views" in Eikon, e.g. TRDPRC_1, BID, ASK, VWAP etc.).
Is there a way to retrieve the fields available for a given security?
I have checked historical pricing service on API Playground and it does not provide fields discovery method. You should request without fields parameter to get all fields. According to Historical Pricing: Time Series Summary (bar), Events (T&S) , all fields are returned by default so requesting recent one data should return all fields as you saw null values.
According to Historical Pricing: Time Series Summary (bar), Events (T&S) , fields are optional; by default all available fields are returned. If you do not specify the fields, you will get all fields of the security. Returned field list order and content can vary depending on the exchange / instrument.
Hope this help.