fix: make input_instruction can be empty string

[ruanyl]

Description

Allow input_instruction to be empty string.

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test.
• New functionality has user manual doc added.
• Commits are signed per the DCO using --signoff.

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/dashboards-assistant/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/dashboards-assistant/backport-2.x
# Create a new branch
git switch --create backport/backport-299-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 52b57e21f667a18c35ff9f913c47d71cbc72e3f2
# Push it to GitHub
git push --set-upstream origin backport/backport-299-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/dashboards-assistant/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-299-to-2.x.