Remove empty elements from exploded character-ngrams output

[davidwendt]

Description

Fixes character_ngrams function to not include empty entries when as_list=False. That is, the exploded view (non-list result) should not contain empty or NA elements.

This PR changes the nvtext::generate_character_ngrams() API to return a lists column instead of a flat strings column. The python code had been converting the return object into lists column and then exploding it if as_list=False. Returning as a list column simplifies the logic and prevents the double conversion. There is almost no impact to the nvtext code since the offsets for the output lists column were already being generated.

All tests were updated to expect the new result. Also changed some exception types from cudf::logic_error to std::invalid_argument as appropriate.

Continues work of abandoned PR #14685

Checklist

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

[davidwendt]

/merge