BUG: Don't supply name to Series when using 'size' with agg/apply
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

Ref: https://github.com/pandas-dev/pandas/pull/39935#discussion_r651989373

Doing e.g.

    print(DataFrame({'a': [1, 2, 3], 'b': [4, 5, 6]}).apply('sum'))
    print(DataFrame({'a': [1, 2, 3], 'b': [4, 5, 6]}).agg('sum'))

produce a Series with `name=None`. This was introduced in 1.3, so does not need a whatsnew.