fix(inputs.mysql): Revert slices declarations with zero initial length

[zak-pawel]

Reverts (mysql.go only): #12371
Fixes: #12398

Pointer to rawBytes := sql.RawBytes{} wrongly used (instead of pointer to slice).
In that case, enforcing slices declarations with zero initial length doesn't make much sense.

[telegraf-tiger]

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[utking]

The slice declaration does not change anything. But how the pointer is used does. A simple example to show that the pointers (for t and vals[i]) have different addresses in memory would be

for i := range items {
    t := sql.RawBytes{}
    vals = append(vals, t)
    valPtrs = append(valPtrs, &t)
    fmt.Printf("%p:%p\n", &t, vals[i])
}

That is, having valPtrs = append(valPtrs, &rawBytes) reverted to use a slice index again would be enough, and would satisfy the linter.