Explicitly require the :post parameter to be present when attempting to create a post

[Oaphi]

closes #1891

The error is likely not reproducible outside of very specific circumstances, but it doesn't mean we shouldn't gracefully handle it instead of outright failing with a server error 500:

Repro is handled by the added test case, so the only part to test here manually if one wants to do that is that normal post creation still works.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.65%. Comparing base (a93b782) to head (e8a8c1a).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	74.70% <100.00%> (+0.04%)	⬆️
helpers	84.91% <ø> (ø)
jobs	80.59% <ø> (ø)
models	90.09% <ø> (ø)
tasks	61.11% <ø> (ø)

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