BUG: fix fill value for gouped sum in case of unobserved categories for string dtype (empty string instead of 0)#61909

jorisvandenbossche merged 4 commits intopandas-dev:mainpandas-dev/pandas:mainfrom

jorisvandenbossche:string-dtype-groupby-sum-observed-false-fillvaluejorisvandenbossche/pandas:string-dtype-groupby-sum-observed-false-fillvalueCopy head branch name to clipboard