Currently, the PathArrayness API uses FieldPath and the constructor of FieldPath will uassert if we give it an invalid FieldPath even if the actual path is valid in MQL, e.g. empty path in MatchExpressions.

We should validate the input FieldPath by calling FieldPath::validateFieldName() and if we return an error, we should just return the conservative guarantee, i.e. input field path is an array.