Fix #135: avoid NPE in JarVersionedRuntimes.getJarClasses for missing version

[elharo]

Fixes #135

JarVersionedRuntimes.getJarClasses(Integer version) called versionedRuntimeMap.get(version).getJarClasses(), which throws a NullPointerException when the requested version is not a key in the map. This is inconsistent with getJarVersionedRuntime(), which safely returns null for missing keys.

This change guards against a missing key and returns null, matching the documented behaviour of the sibling accessor.

Added a unit test JarVersionedRuntimesTest that reproduces the NPE (fails before the fix, passes after):

• getJarClassesForExistingVersion verifies the existing key still returns the expected JarClasses
• getJarClassesForMissingVersionReturnsNull verifies a missing version returns null instead of throwing