Fix Strings.CI.indexOf returning out-of-range index for empty search

[alhudz]

1. CiStrings.indexOf (the case-insensitive Strings.CI.indexOf) bails out only when startPos > endLimit, where endLimit = str.length() - searchStr.length() + 1.
2. For an empty search endLimit is str.length() + 1, so a startPos of exactly str.length() + 1 slips past the guard and the empty-search branch returns startPos unchanged.

Repro: Strings.CI.indexOf("abc", "", 4)
Expected: -1 (the class Javadoc documents out-of-range start positions as -1, e.g. Strings.CI.indexOf("abc", "", 9) = -1)
Actual: 4, an index one past the end of a length-3 string

The case-sensitive CS.indexOf delegates to String.indexOf and clamps, so only the hand-rolled CI implementation is affected. Changed the guard to startPos >= endLimit; -1 is returned once startPos passes the last valid empty-match position (str.length()), and every documented example is unchanged.

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[garydgregory]

@alhudz
FYI: I cleaned up the test a bit to make it clear we are testing for the specific edge case of an "empty search". Rebuilding...