BUG: Don't supply name to Series when using 'size' with agg/apply

[rhshadrach]

• Ensure all linting tests pass, see here for how to run them

Ref: #39935 (comment)

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]

lgtm merge and backport on greenish

[rhshadrach]

@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) =

[simonjayhawkins]

@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.

[simonjayhawkins]

Thanks @rhshadrach