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.
@jreback greenish; failure is unrelated

> FAILED pandas/tests/indexes/numeric/test_numeric.py::TestUInt64Index::test_constructor_does_not_cast_to_float
= 1 failed, 170123 passed, 6374 skipped, 874 xfailed, 6 xpassed, 352 warnings in 1414.92s (0:23:34) =
 > @jreback greenish; failure is unrelated
> 
> > FAILED pandas/tests/indexes/numeric/test_numeric.py::TestUInt64Index::test_constructor_does_not_cast_to_float
> > = 1 failed, 170123 passed, 6374 skipped, 874 xfailed, 6 xpassed, 352 warnings in 1414.92s (0:23:34) =

now fixed. Thanks @rhshadrach 