Refuse a present-but-blank option with OpenKaValidationError. An omitted one
means the default (deu, any version, the binary's own traineddata); a blank one
is a mistake, and accepting it stamped a provenance naming no language
(tesseract-5.3.4+) or failed late with an empty slot in the message. Both
perceiver constructors call this first.
Refuse a present-but-blank option with
OpenKaValidationError. An omitted one means the default (deu, any version, the binary's own traineddata); a blank one is a mistake, and accepting it stamped a provenance naming no language (tesseract-5.3.4+) or failed late with an empty slot in the message. Both perceiver constructors call this first.