`search` should create an empty file when no triples are found

Nobody has claimed this yet.

We had a long-standing issue in a pipeline where we are using OAK search (`runoak -i wikidata: search -O rdf --output out.txt`), and everything kept working for months, while the pipeline was broken (for a reason unrelated to oak, the search terms where malformed). The problem was that the `search`, when it does not find anything, does not produce any output. So basically, the pipeline kept using the old output (`out.txt`), because it was never overwritten, which meant we only noticed the error months later when a new user tried running the pipeline.

Can I suggest that `search` creates an empty file when it "does to find anything" to prevent such errors in the future?

cc @sabrinatoro (FYI)

Start by reproducing the `runoak -i wikidata: search -O rdf --output out.txt` command with search terms that return no triples. Trace the `search` CLI entry point and its `--output` handling, then verify that a no-result run creates or truncates `out.txt` and does not retain stale content.

Written by the indexing model from the issue text.