Fix make_column_from_scalar for all-null strings column

[davidwendt]

Description

Fixes the cudf::make_column_from_scalar for an invalid cudf::string_scalar to return a column with children. Some libcudf APIs will not work with a strings column with no children. This condition would be rare enough that additional logic for checking no children in these places would be a performance and maintenance issue.
This also greatly simplifies the make_column_from_scalar specialization logic for strings.

Closes #11756

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

Base: 87.40% // Head: 87.52% // Increases project coverage by +0.11% 🎉

Coverage data is based on head (b4ce285) compared to base (f72c4ce).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11807      +/-   ##
================================================
+ Coverage         87.40%   87.52%   +0.11%     
================================================
  Files               133      133              
  Lines             21833    21807      -26     
================================================
+ Hits              19084    19086       +2     
+ Misses             2749     2721      -28     

Impacted Files	Coverage Δ
python/cudf/cudf/core/udf/__init__.py	50.00% <ø> (ø)
python/cudf/cudf/utils/ioutils.py	79.47% <ø> (ø)
...thon/dask_cudf/dask_cudf/tests/test_distributed.py	18.86% <ø> (+4.94%)	⬆️
python/cudf/cudf/__init__.py	90.69% <100.00%> (ø)
python/cudf/cudf/core/scalar.py	90.52% <100.00%> (+1.25%)	⬆️
python/dask_cudf/dask_cudf/sorting.py	93.29% <100.00%> (+2.11%)	⬆️
python/strings_udf/strings_udf/__init__.py	96.87% <100.00%> (ø)
python/strings_udf/strings_udf/_typing.py	94.73% <0.00%> (-1.06%)	⬇️
python/cudf/cudf/testing/dataset_generator.py	72.83% <0.00%> (-0.42%)	⬇️
python/cudf/cudf/core/column/numerical.py	95.21% <0.00%> (-0.29%)	⬇️
... and 1 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[PointKernel]

Is there a test exercising this fix?

[davidwendt]

Is there a test exercising this fix?

Yes. There are existing tests. Here are two of them:

cudf/cpp/tests/column/factories_test.cpp

Lines 417 to 437 in 5a4afec

	TEST_F(ColumnFactoryTest, FromStringScalar)
	{
	cudf::string_scalar value("hello");
	auto column = cudf::make_column_from_scalar(value, 1);
	EXPECT_EQ(1, column->size());
	EXPECT_EQ(column->type(), value.type());
	EXPECT_EQ(0, column->null_count());
	EXPECT_FALSE(column->nullable());
	EXPECT_FALSE(column->has_nulls());
	}
	TEST_F(ColumnFactoryTest, FromNullStringScalar)
	{
	cudf::string_scalar value("", false);
	auto column = cudf::make_column_from_scalar(value, 2);
	EXPECT_EQ(2, column->size());
	EXPECT_EQ(column->type(), value.type());
	EXPECT_EQ(2, column->null_count());
	EXPECT_TRUE(column->nullable());
	EXPECT_TRUE(column->has_nulls());
	}

[PointKernel]

LGTM. Thanks!

[davidwendt]

@gpucibot merge