[iamralch]

FilterClause::parse read a blank string as FilterClauseError::Blank, so every caller special-cased it before parsing. OrderByClause::parse, by contrast, reads a blank ordering as the empty clause and needs no such care. AIP-160 reads a blank filter as no filter as well.

Now both read the same way:

let filter = FilterClause::parse(request.filter)          // "" -> the empty filter
    .and_then(|filter| filter.resolve(&COLUMNS))
    .map_err(...)?;
let order_by = OrderByClause::parse(request.order_by)     // "" -> the empty clause
    .and_then(|order_by| order_by.resolve(&COLUMNS))
    .map_err(...)?;

• sqlx-query-cel: a blank filter parses to the empty FilterClause (now Default). It resolves against any mapping and renders as an empty WhereClause. FilterClauseError::Blank is removed.
• sqlx-query: WhereClause gains Default and is_empty(), and and/or return the other side when one is empty. Before this they rendered () AND (...). The composer leaves the slot empty for an empty clause, and a query with no where slot accepts one.

Breaking: FilterClauseError::Blank is gone, and FilterClause::parse returns Ok for a blank string.

Tests: and/or with an empty side; an empty filter dropping the slot, needing no slot, and leaving a cursor's comparison alone; a blank filter parsing, resolving and rendering empty; and a live SQLite query where a blank filter returns every row. The README's Filtering section is updated.