REGR: fillna not filling NaNs after pivot without explicitly listing pivot values on 1.1.x
- [ ] xref #36495

yet another alternative to fix directly on 1.1.x pending further discussion in #34407 on how to resolve the issues on master.

cc @jorisvandenbossche @jbrockmendel 

This won't close issue until fix on master. could copy test to master with xfail (and would need to sync release notes if we merge this direct to 1.1.x)