[fix] return results for a query made only of negated terms - #806

A negated field filter on its own returns every other document: -title:golang lists everything whose title doesn't contain golang. A negated plain term on its own returns nothing, and neither does a query made only of negated terms:

(These come from three indexed pages: go.dev with "golang language", rust-lang.org with "rust language" and python.org with "python language".)

Bleve handles a boolean query that only has must-not clauses fine; it adds a match-all as the required part. querybuilder.Build adds a required clause in two places, though:

• With a single term that isn't field specific, it adds the base URL boost as a should clause. When the term is negated there's no must clause, so Bleve makes the should clause required, and only documents whose URL starts with golang (or with -golang, since the raw token is used) can match.
• With several terms, it wraps the positive terms in Disjunction(Conjunction(positive terms...), phrase(full query)). When every term is negated, the conjunction is empty, so the whole disjunction depends on the phrase -golang -python matching.

The fix only adds each of those when there's a positive term to go with it (len(qs) > 0 / len(qs) == 1). Queries with at least one positive term build exactly the same query as before.

Tests:

• TestSearchOnlyNegatedTerms in server/indexer/indexer_test.go indexes the three pages above and checks the table. On main it fails for -golang, -"golang language" and -golang -python.
• Test_build_negated_word now also checks that -golang gets no should clause.
• go test -race ./... passes (with a placeholder server/static/app for the embed), and golangci-lint run ./server/indexer/... reports 0 issues.