MultiPartParser skips fields when sending empty files

[27justin]

Describe the bug
MultiPartParser does not parse the request body correctly when an empty file is transmitted as the first member of the body.

To Reproduce

1. Create a POST Endpoint that reads out the multipart body.
2. Create a sample HTML page that has a <input type="file" /> as the first input in a form
3. Add more of whatever types
4. Dispatch the form without supplying a file
5. Endpoint reads out no parameters

Expected behavior
The MultiPartParser should return the request body with the parameters set.
Currently, MultiPartParser::parse returns -1 with an empty <input type="file" />

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Linux
• Browser: Firefox
• Version: 133.0.3

The bug also occurs on chrome.

Additional context
The implementation of MultiPartParser::parseEntity is faulty in that it expects each multi-part file to have a filename.
When sending a form that has no filename set, the parser stops parsing subsequent fields.
The offending code is the conditional at MultiPart.cc:214

Here's an excerpt of the failing request body:

-----------------------------105238891342303886541107959621
Content-Disposition: form-data; name="cover"; filename=""
Content-Type: application/octet-stream


-----------------------------105238891342303886541107959621
Content-Disposition: form-data; name="abstract"

test
-----------------------------105238891342303886541107959621
Content-Disposition: form-data; name="language"

test
-----------------------------105238891342303886541107959621--