Allow using Vec<Option<String>> as part of FromQueryResult

[ctron]

Assuming the following struct:

#[derive(Debug, FromQueryResult)]
pub struct Node {
    pub foo: String,
    pub bar: Vec<Option<String>>,
}

Compilation fails with with:

error[E0277]: the trait bound `Vec<std::option::Option<std::string::String>>: TryGetable` is not satisfied
  --> xx
   |
23 | #[derive(Debug, FromQueryResult)]
   |                 ^^^^^^^^^^^^^^^ the trait `TryGetable` is not implemented for `Vec<std::option::Option<std::string::String>>`
   |
   = help: the following other types implement trait `TryGetable`:
             Vec<Braced>
             Vec<Hyphenated>
             Vec<JsonValue>
             Vec<NaiveDate>
             Vec<NaiveDateTime>
             Vec<NaiveTime>
             Vec<OffsetDateTime>
             Vec<PrimitiveDateTime>
           and 21 others