CMake: handle hunspell-has-no-dictionaries
If there are **no** dictionaries for hunspell, then the AVAILABLE DICTIONARIES line is followed by no output (no lines listing dictionaries). Then the match fails, so CMAKE_MATCH_1 is empty, and substituting it in the REGEX REPLACE command triggers a CMake error like this: CMake Error at cmake/modules/FindHunspell.cmake:54 (string): string sub-command REGEX, mode REPLACE needs at least 6 arguments total to command. Quote it, so that at least the empty string is passed in when no dictionaries are available.
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