Skip multipart parts with empty Content-Type instead of raising

[magicmark]

Multipart HTTP streams may contain empty parts (no headers, no body):

NO header fields are actually required in body parts. A body part that starts with a blank line, therefore, is allowed
~ https://www.rfc-editor.org/info/rfc2046/#section-5.1

(In our case, we see this (I think) due to being passed through multiple proxies which unpack and pack parts back together)

Anyway our current handling is too strict — it required every part to have Content-Type: application/json instead of just skipping empty ones.

Before

When this happens, applications see traceback like this:

gql.transport.exceptions.TransportProtocolError: Unexpected part content-type: . Expected 'application/json'.
Unexpected part content-type: . Expected 'application/json'.

After

✨ empty parts are ignored; no errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (1ff86bc).
⚠️ Report is 61 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #598    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3325   +417     
==========================================
+ Hits          2908      3325   +417     

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