Correctly handle 0 length strings in conio

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Checklist

Thanks a lot for your contribution!
Please tick off the following:

• Tests run successfully (i.e. make test)
  • [*] using the CC65 compiler
  • [* using the LLVM/Clang compiler
• Doxygen style tags are used for public API
• Source code is properly formatted; run e.g. clang-format -i <file>
• [XI agree to the License of this repository

• • Please note that there appears to be a single test failure however that may be due to my atypical Xemu usage. It also is in an unrelated test.

Also, I removed the undocumented behavior note on cputncxy for a zero length string. 0 length will now work as expected, eg: no output.