Missing ROOT key causes NPE in JarClassesAnalysis for malformed MR-JARs

In `JarClassesAnalysis.java:153-155`:

```java
JarVersionedRuntime rootContentVersionedRuntime = runtimeVersionsMap.remove(ROOT);
jarData.setRootEntries(rootContentVersionedRuntime.getEntries()); // NPE if no ROOT
```

If a malformed multi-release JAR has no root-level entries (every entry is under `META-INF/versions/N/`), `runtimeVersionsMap` has no `ROOT` key, `remove(ROOT)` returns `null`, and `.getEntries()` throws a NullPointerException.

**Fix**: Guard against null:
```java
JarVersionedRuntime rootContent = runtimeVersionsMap.remove(ROOT);
if (rootContent != null) {
jarData.setRootEntries(rootContent.getEntries());
}
```

Start in JarClassesAnalysis.java at lines 153-155 and inspect how the ROOT entry is removed from runtimeVersionsMap. Exercise the analysis with a malformed multi-release JAR containing only META-INF/versions/N/ entries, and confirm processing completes without a NullPointerException when ROOT is absent.

Written by the indexing model from the issue text.