Add None type check for content_type

[codingjoe]

If the Content-Type header is not set, content_type will be None.
We should not perform a string operation on None.

AttributeError: 'NoneType' object has no attribute 'startswith'
  File "laas/server.py", line 47, in middleware_handler
    return await handler(request)
  File "laas/server.py", line 22, in handle
    data = await request.post()
  File "aiohttp/web_request.py", line 412, in post
    if content_type.startswith('text/'):

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new entry to CHANGES.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using #issue_number format at the end of changelog message. Use Pull Request number if there are no issues for PR or PR covers the issue only partially.

[kxepal]

Looks like duplicate of #1743

[codingjoe]

@kxepal thx it is

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