CVE-2017-20166: Ecto lacks a protection mechanism

The vulnerability stems from the reverted commit 8e1b378 which modified query_for_get_by to handle nil values with is_nil. The patch diff shows this function was responsible for processing WHERE clauses in get_by queries. By automatically converting {key, nil} tuples to is_nil(field(x, ^key)) without proper validation, it circumvented Ecto's security measures that normally require explicit is_nil usage. This matches the advisory's description of lacking protection mechanisms between is_nil and raise, and the revert commit message explicitly cites security implications for nil handling.