[ktexttohtml] Fix invalid string access
If someone passes an empty string to KTextToHTMLHelper (like ktexttohtmltest.cpp:34 does) accessing any index of the string will blow up.
Check for an empty string to avoid that.
If someone passes an empty string to KTextToHTMLHelper (like ktexttohtmltest.cpp:34 does) accessing any index of the string will blow up.
Check for an empty string to avoid that.