Empty PostgreSQL ARRAY should be printed with a cast

[tisonkun]

Otherwise, it will fail with "cannot determine type of empty array".

For example, an empty string vec will be converted to ARRAY [] but PG needs ARRAY[]::TEXT[] or things like that for understanding the type.

[tisonkun]

Or use {} as reported at graphile/crystal#406.

[tisonkun]

I provide a fix in #854, but found another issue that if we render a Vec<Uuid> to array, it would be like ARRAY[ '<uuid>' ] that would be inferred as TEXT[]. So we may instead render a cast for all array value, i.e., ARRAY ['<uuid>'] :: UUID[], and ditto to all array types.

[tisonkun]

ARRAY [''] :: UUID[]

This would failed on type mismatch like String may be TEXT or VARCHAR. Then I'd like to keep #854 while other types can be cast with SimpleExpr::cast_as.

For example:

// ids: Vec<Uuid>
let expr: SimpleExpr = request.ids.clone().into();
expr.cast_as(IdenStr("UUID[]"))