Monthly CheckIn - Media Links optional fix

[Disura-Randunu]

Purpose

The purpose of this PR is to fix #218

Goals

Media Links must be optional when submitting a monthly check-in.

Approach

Modified validation schema of monthly checkin to handle optional media links

Screenshots

Checklist

• [x] This PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.
• [x] I have read and understood the development best practices guidelines ( http://bit.ly/sef-best-practices )
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation

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