[MNG-6907] fix endless loop in StringSearchModelInterpolator

[Bananeweizen]

Avoid endless recursion by checking for equality between current parameter and recursion parameter. Although it seems unreasonable to have Java classes, where the superclass is identical to the current class, I've experienced this in practice.

I've verified there are no other code locations using a similar recursion pattern in StringSearchModelInterpolator or its successor StringVisitorModelInterpolator.

• Make sure there is a JIRA issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a JIRA issue. Your pull request should address just this issue, without pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [MNG-XXX] - Fixes bug in ApproximateQuantiles,
  where you replace MNG-XXX with the appropriate JIRA issue. Best practice
  is to use the JIRA issue title in the pull request title and in the first line of the
  commit message.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean verify to make sure basic checks pass. A more thorough check will
  be performed on your pull request automatically.
• You have run the Core IT successfully.

Core IT has not been run. Documentation says to use profile "run-tis", but there is no such profile.

To make clear that you license your contribution under
the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

• In any other case, please file an Apache Individual Contributor License Agreement.

[khmarbaise]

First let us check what CI says...

https://builds.apache.org/job/maven-box/job/maven/job/Bananeweizen-MNG-6907/

[rfscholte]

This doesn't look correct to me. Something like root().firstNode().firstNode().firstNode() looks valid to me.
Hence, this PR requires a unittest to cover such a case as well.

[slachiewicz]

SearchStringInterpreter is no longer in use in Maven Core and in 3.6.3 shouldn't be injected

[rfscholte]

So this means that a plugin that needs it, will require at least Maven 3.6.2 at compile time (or use reflection) and at runtime it must verify if StringVisitorModelInterpolator is avaible. If not, keep using the StringSearchModelInterpolator.
If that is the idea, one should close this PR and MNG-6907