Handle empty query stems safely in query side stemming

[SahilKumar75]

Implements issue #33540.

This PR handles empty-stem query terms in query-side stemming with a narrow scope.

Changes:

• Keep phrase and non-text operator behavior unchanged.
• Safely drop empty-stem terms only in query-side AND/WeakAND contexts when at least one sibling term remains.
• Do not allow the whole query to collapse to empty.
• Preserve connectivity-sensitive terms.

Regression tests added for:

• Query.And: drop empty-stem term when another term remains.
• Query.And: do not collapse to empty.
• userInput path: drop empty-stem term when safe.
• userInput path: do not collapse to empty.
• Phrase behavior unchanged.
• Non-text operator behavior unchanged.

Verification:

• mvn -pl container-search -Dtest=StemmingSearcherTestCase test
• mvn -pl container-search -Dtest=StemmingSearcherTestCase,WeakAndTestCase,StopwordTestCase test

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's
source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[SahilKumar75]

Buildkite is still running.

The Link checker failure appears to be workflow/infrastructure-related, not code-related:
actions/create-github-app-token@v2 failed with [@octokit/auth-app] appId option is required after htmlproofer completed successfully.

Could a maintainer please rerun the Link checker check when convenient?

[SahilKumar75]

@bratseth @hmusum could you review when convenient? Buildkite is still running; Link checker failed due workflow infra and may need maintainer rerun.

[bratseth]

Consider these simplifications, otherwise LGTM!