[server] views: comments: Check using get(), not 'in'

[ix5]

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)
  

What changes does this Pull Request introduce?

Previously, if a dict item was supplied by the JS client but contained an invalid value such as null, the server would only check if the dict key was provided, not if its value was falsy.

>>> mydict = dict(a='hi', b=None)
>>> 'b' in mydict
True
>>> 'b' not in mydict
False

Instead, use .get() and check if the value is empty. Then, for instance, attempt to parse thread uris if payload title is invalid instead of simply accepting null as thread title.

Also add a test to check this behavior.

Why is this necessary?

Fixes: #874