fix #18128 rfind on empty needle returns rightmost index

[n5m]

Fix #18128

All the relevant test cases discussed in the linked issue pass now

import strutils

doAssert "abc".find("") == 0
doAssert "abc".rfind("") == 3

doAssert "abc".find("") == 0
doAssert "abc".rfind("") == 3
doAssert "abc".rfind("c") == 2
doAssert "abc".rfind("bc") == 1
doAssert "abc".rfind("abc") == 0

doAssert "abc".find("") == 0
doAssert "abc".find("a") == 0
doAssert "abc".find("ab") == 0
doAssert "abc".find("abc") == 0

[github-actions]

Thanks for your hard work on this PR!
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