Drop trailing empty token from splitByWholeSeparator

[alhudz]

Repro: StringUtils.splitByWholeSeparator("a:b:", ":") returns ["a", "b", ""]; expected ["a", "b"]. The sibling StringUtils.split("a:b:", ":") already returns ["a", "b"].

Cause: in splitByWholeSeparatorWorker the no-more-separator branch adds str.substring(beg) unconditionally. When the input ends on a separator, beg == len, so an empty trailing token is appended. Leading and adjacent separators are already collapsed in non-preserve mode, so only the trailing case leaked, which disagrees with the documented "adjacent separators are treated as one separator".

Fix: in non-preserve mode only add the trailing token when it is non-empty (beg < len). splitByWholeSeparatorPreserveAllTokens keeps the trailing empty token as before.

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[garydgregory]

@alhudz
Please see my comment.

[garydgregory]

Please add a new @ParameterizedTest method for these new assertions.

[alhudz]

Done. Moved them into a new @ParameterizedTest, testSplitByWholeStringDropsTrailingEmpty, driven by @CsvSource. Each row also asserts the preserve-all-tokens variant still keeps the trailing empty token. Confirmed it fails on every row without the worker guard.