Split now supports empty delimiter

Split was not checking the delimiter's size and would run forever
when delimiter was empty.
Now, in case of an empty delimiter, we just split all characters.

src/utils/string.hpp

@@ -209,8 +209,13 @@ std::vector<TString, TAllocator> *Split(std::vector<TString, TAllocator> *out, c
   if (src.empty()) return out;
   size_t index = 0;
   while (splits < 0 || splits-- != 0) {
-    auto n = src.find(delimiter, index);
-    if (n == std::string::npos) break;
+    size_t n = 0;
+    if (delimiter.empty()) {  // Special case where we just return characters
+      n = index + 1;
+    } else {
+      n = src.find(delimiter, index);
+    }
+    if (n >= src.size()) break;
     out->emplace_back(src.substr(index, n - index));
     index = n + delimiter.size();
   }