Fix test in ExceptionTranslatorIT

[pascalgrimaud]

As my locale is french, it failed with this message:

java.lang.AssertionError: JSON path "$.fieldErrors.[0].message" expected:<must not be null> but was:<ne doit pas être nul>
Expected :must not be null
Actual   :ne doit pas être nul

I was able to reproduce by generating a JHipster project with Webflux + noi18n

[pascalgrimaud]

cc @gzsombor @murdos @juliensadaoui as you're our Webflux Champions !