BUG: copying from Jupyter table to excel misaligns structure
- [x] closes #12147 

when copying a table from Jupyter to Excel the column header row is translated to the left because there is no value to copy.

![Screen Shot 2021-03-10 at 19 53 28](https://user-images.githubusercontent.com/24256554/110682247-019fc180-81db-11eb-81a4-f8d71b08181e.png)

this PR puts a non-breaking space in the empty cell so that structure is preserved.

![Screen Shot 2021-03-10 at 19 55 41](https://user-images.githubusercontent.com/24256554/110682369-2431da80-81db-11eb-9198-5e13661805cf.png)

my excel above is quite old, but this effect is worse on more recent excels since I believe they merge cells which have a rowspan (in the multiindex column headers) or a colspan (in a multiindex index labels). With columns merging in wrong place it is a bit of a nuisance. 