string_util: Get rid of separate resize() in CPToUTF16(), UTF16ToUTF8(),...

string_util: Get rid of separate resize() in CPToUTF16(), UTF16ToUTF8(), CodeToUTF8() and UTF8ToUTF16()

There's no need to perform the resize separately here, since the
constructor allows presizing the buffer.

Also move the empty string check before the construction of the string
to make the early out more straightforward.