TST: to_json keeps column info with empty dataframe (#7445)
 - [x] closes #7445
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

Closed by #9805

the original issue has sufficient tests AFICT. what does this add? ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14893?src=pr) is 85.31% (diff: 100%)


> No coverage report found for **master** at e7df751.

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e7df751...fe2b110](https://codecov.io/gh/pandas-dev/pandas/compare/e7df7516ff44185ded46f89a463169c997ae0662...fe2b110939c0ca40f7889491ad61ad4abbab483a?src=pr) Ah, yes, that is maybe true. The original PR has roundtrip tests for empty frames, while this one adds an explicit test of the output format in such a case. So that's maybe slightly useful to add, but in any case, if we keep it, would be good to have them at the same place Agreed, makes sense to move it to `test_frame_empty`. Doesn't look like this specific case is tested there. I'll make the change later tonight.  thanks!