string_util: Use emplace_back() in SplitString() instead of push_back()

This is equivalent to doing:

push_back(std::string(""));

which is likely not to cause issues, assuming a decent std::string
implementation with small-string optimizations implemented in its
design, however it's still a little unnecessary to copy that buffer
regardless. Instead, we can use emplace_back() to directly construct the
empty string within the std::vector instance, eliminating any possible
overhead from the copy.