Made event invite emails show placeholder text with optional fields

[vkoves]

Description

This resolves #417, where event invite emails shows nothing if fields are empty. Here's a screenshot of the preview email:

Type of Pull Request

Based on the contributor's guide, this PR is of type:

• [ ] Development (feature-branch -> dev)
• [ ] Hotfix (hotfix-branch -> master)
• [ ] Release (release-branch -> master)
• [x] Other

Requestor Checklist

Requestor: Put an x in all that apply. You can check boxes after the PR has been made.

Reviewer: If you see an item that is not checked that you believe should be, comment on that as part of your review.

• [ ] Code Quality: I have written tests to ensure that my changes work and handle edge cases
• [ ] Code Quality: I have documented my changes thoroughly (using JSDoc in Javascript)
• [x] Process: I have linked relevant issues, marking issues that this PR resolves
• [x] Process: I have requested at least as many reviews required for this PR type (2 or 3)
• [x] Process: I have added this PR to the relevant quarterly milestone
• [x] Process: I have tested this PR locally and verified it does what it should

How This Has Been Tested

Tested manually in the email previews.