Solve issue with instance messages and `requirements` empty field

[nesitor]

Problem: When a user send an instance message with the field requirements to null, the message is rejected.

Solution: Check if the requirements field have something relevant inside.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.37%. Comparing base (38f8fa7) to head (e79aa8a). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #599 +/- ## ======================================= Coverage 94.37% 94.37% ======================================= Files 91 91 Lines 4910 4910 Branches 677 677 ======================================= Hits 4634 4634 Misses 241 241 Partials 35 35 ```

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