In SonarQube, We have a rule, S2699 - Tests should include assertions. This is not supported for WebTestClient as assertion happens inside webtestclient.
Rule S2699 has a parameter customAssertionMethods where you can configure the signature of assert methods for additional frameworks. This parameter was added few months ago, so you need to have reasonably recent version of Java analyzer installed (it is for sure available on latest 7.9 LTS).