I have provided the solution in the following post. I used to get the NULL value too.
Solution: equity_history() function doesn’t work - Programming / NSE Python API - Unofficed
Additionally, I also observed that the date-range is too long. Try to keep it in between 2 months or so.
You can verify the max date range from the NSE link: SBI Life Insurance Company Limited Share Price Today, Stock Price, Live NSE News, Quotes, Tips – NSE India
Even if you select 6 months date range, the result is limited to roughly 2 and half months.
Hope this helps.