return empty string if secondary hashtag is undefined/null

[suzit-10]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Related Issue

Example: Fixes #123

Describe this PR

A brief description of how this solves the issue.

Screenshots

Please provide screenshots of the change.

Alternative Approaches Considered

Did you attempt any other approaches that are not documented in code?

Review Guide

Notes for the reviewer. How to test this change?

Checklist before requesting a review

• 📖 Read the Tasking Manager Contributing Guide: https://github.com/hotosm/tasking-manager/blob/develop/docs/developers/contributing.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.
• 🔠 Does this PR introduce or change any environment variables? If so, make sure to specify this change in the description.

[optional] What gif best describes this PR or how it makes you feel?