Replace make_empty_strings_column with make_empty_column

[davidwendt]

Closes #8352

This PR replaces the cudf::strings::detail::make_empty_strings_column call with the existing cudf::make_empty_column(cudf::data_type{cudf::type_id::STRING}) instead.

This allowed also removing the include of cudf/strings/detail/utilities.hpp from some source files as well.

No function has changed. The make_empty_strings_column detail utility is removed.

[ttnghia]

I wonder why don't we call make_empty_column(cudf::type_id) instead of make_empty_column(cudf::data_type{cudf::type_id})?

[galipremsagar]

rerun tests

[codereport]

lgtm

[codecov]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.08    #8435   +/-   ##
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  Hits                    ?    14828           
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[harrism]

@gpucibot merge