... to stay backward compatible with previous behavior.
About detection failure, our in-code documentation says:
"@return name of charset on success and "" on failure or pure ascii."
This behavior had been broken by commit 3a518c0, which returned NULL
instead. Our command-line tool was the first victim, segfaulting on
ASCII files.
This fixes the following warning when including uchardet.h in C source,
built with -Wstrict-prototypes:
`uchardet.h:52:1: warning: function declaration isn't a prototype`
