std.mem.indexOfPos should return start_index when needle length is zero

[mbolis]

Zig Version

0.9.0-dev.1712+9836f1b2f

Steps to Reproduce

Search for an empty slice inside another:

const std = @import("std");
const indexOfPos = std.mem.indexOfPos;
const expect = std.testing.expect;

test "should return start index" {
    const index = indexOfPos(u8, "abracadabra", 5, "") orelse unreachable;
    try expect(index == 5); // fails
}

Expected Behavior

I expect an empty slice to be always found exactly at the specified start index.

Actual Behavior

The indexOfPos function always returns 0 if the needle has no length.