Fix querier panic on tag value autocomplete with only metadata-only intrinsic conditions

[zalegrala]

What this PR does:

Fixes a querier panic when every condition in a tag value autocomplete request is a metadata-only intrinsic. SearchTagValuesV2 for span:id with query {trace:id="..."} crashes the process:

panic: runtime error: index out of range [0] with length 0
  pkg/parquetquery.(*JoinIterator).Next            iters.go:1052
  vparquet5.(*backendBlock).FetchTagValues.func2   block_autocomplete.go:281
  querier.(*Querier).SearchTagValuesV2Handler      http.go:317

span:id, span:startTime, trace:id and trace:start are skipped with a bare continue and contribute no iterator. When every condition is one of those, the span stage yields nil and createDistinctTraceIterator builds a JoinIterator over an empty slice, which indexes j.peeks[0] unconditionally.

NewLeftJoinIterator already rejects this shape; NewJoinIterator doesn't, and giving it an error return would touch ~45 call sites. So this fixes the two places that produce the degenerate input:

• createDistinctTraceIterator returns nil, nil when it has nothing to join, matching createDistinctAttributeIterator.
• The four autocompleteIter call sites skip a nil iterator. Required, not defensive — without it the same request panics on WAL blocks at wal_block.go:876.

Applied to vparquet3/4/5; identical code, all three reproduce. Behaviour change is limited to the crashing case: these requests now return no values, as these intrinsics already do elsewhere in this path.

Separately, these conditions are dropped rather than applied, so {trace:id="..."} returns tag values from other traces. Tracked in #7706; out of scope here.

Which issue(s) this PR fixes:

None filed; found from a production panic.

Checklist

• Tests updated — cases in TestFetchTagValues plus a new TestWalBlockFetchTagValuesMetadataOnlyIntrinsics (WAL autocomplete had no coverage); all panic without the fix
• Documentation added — n/a
• Changelog entry added under .chloggen/