fix: omit empty rank conditions for sort-only scopes

[sean0407]

Summary

ScopeOptions#conditions
  discard sort_only features
+ if no filter expressions remain: return nil
  otherwise: build the OR predicate

Fixes #598. When every search feature is sort_only, the empty predicate currently reaches .where and generates invalid SQL with a bare WHERE.

Evidence

• Before: The focused regression example failed with PG::SyntaxError at WHERE ).
• After: The focused example passed (1 example, 0 failures); spec/integration/pg_search_spec.rb passed (100 examples, 0 failures).
• bundle exec standardrb lib/pg_search/scope_options.rb spec/integration/pg_search_spec.rb passed.
• Patch policy passed: 2 files, 16 changed lines, no binary changes.

Merge Danger

Door: Two-way

Blast Radius: Narrow

Only scopes with no non-sort_only conditions now omit the empty predicate. Scopes with filter expressions continue to build the same OR condition.