BUG: read_excel forward-filling MI names
- [x] closes #34673
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Added test with OP's issue as a sheet in the style of existing sheets in `tests/io/data/excel/testmultiindex.*`

(the same as the "both_name" sheet, but with the cell below the MI name empty)

New sheet looks like:
<img width="393" alt="Screen Shot 2020-12-15 at 9 50 02 PM" src="https://user-images.githubusercontent.com/37011898/102298700-961dbf00-3f1f-11eb-9c98-034268ad9d68.png">

In master without this change, the empty cell was being filled with the index name `ilvl2`
Thanks @mzeitlin11 !