[Bug] replace returns wrong result for empty-string search

[andygrove]

Describe the bug

replace(str, search, replacement) diverges from Spark when search is the empty literal string. Spark returns str unchanged (short-circuit on search.numBytes == 0 in StringReplace.eval). Comet delegates to DataFusion's replace, which inserts replacement between every character and at both boundaries.

How to Reproduce

SELECT replace('hello', '', 'x');

Engine	Result
Spark	hello
Comet (DataFusion)	xhxexlxlxox

Additional context

Surfaced by the string-expressions audit (#4461) follow-up. Issue #3344 covered the same divergence but the body had the expected/actual values swapped, leading to it being closed as already-fixed. This issue restates the divergence with the correct direction.

Workaround: the audit-comet-expression follow-up marks replace as Incompatible(Some(reason)) only when searchExpr is a literal empty string, so the dispatcher falls back to Spark for that specific case unless the user opts in with spark.comet.expression.StringReplace.allowIncompatible=true.