ContentTypeHeaderMatcher dx tests broken with some nginx configs

Problem/Motivation

Tests that rely on the behavior of ContentTypeHeaderMatcher's content type check can fail in some nginx configurations. I'm marking this as minor because I don't think we really "support" running tests on nginx but at the same time this test failure exposes some inconsistent behaviors for rest resources when running on nginx where the improved DX from #2811133: Confusing error response set by ContentTypeHeaderMatcher when forgetting the Content-Type request header doesn't work as expected.

The problem comes from the fact that when using fast cgi configurations based on nginx example you will end up alwayspassing a content type to php regardless of whether the client supplied it. This causes the has check to fail say "yeah there's a header!" even though there really isn't.

nginx fastcgi reference: https://www.nginx.com/resources/wiki/start/topics/examples/full/#fastcgi...

Proposed resolution

Convert the check from has to a truthy test on the value. "" should is functionally the same as not existing so this should be fine.

Remaining tasks

- Patch
- Review

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Release notes snippet