[fix](function) Split UTF-8 strings correctly with empty regexp

[Mryange]

split_by_regexp advanced one byte at a time when the pattern was empty, which split multibyte UTF-8 characters into invalid array elements and could truncate a character when a limit was specified. Root cause: the empty-pattern path bypassed RE2 and treated each byte as a complete character. The implementation now advances by the current UTF-8 code point length while preserving the existing byte-based behavior for ASCII input. Regression coverage verifies Chinese and emoji input with and without a split limit.

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[hello-stephen]

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