Literal Type Test for [^^xsd:string] always returns 0 results

[elrayle]

Description

Using [^^xsd:string] to limit all results to only string object values returns 0 results.

Expected behavior

The results should include strings but not other types.

Actual behavior

Returns 0 results

Test for this issue

Branch: https://github.com/samvera-labs/ldpath/tree/string_results_only

Test:
https://github.com/samvera-labs/ldpath/blob/string_results_only/spec/ldpath_program_spec.rb#L74

Branch Comparison: https://github.com/samvera-labs/ldpath/compare/string_results_only