isClassLoadable answers false on threads without a context classloader

[LuciferYang]

Describe the bug

ReflectUtils.isClassLoadable defaults its loader to the thread context classloader:

def isClassLoadable(
    className: String,
    cl: ClassLoader = Thread.currentThread().getContextClassLoader): Boolean = ...

On a thread whose context classloader is null, DynClasses.builder().loader(null) resolves the name with Class.forName(name, true, null), which delegates to the bootstrap loader. The bootstrap loader sees only JDK classes, so the probe answers false for every application class, including Kyuubi's own, even when that class is loadable through the loader that defined ReflectUtils.

A thread can carry a null context classloader in embedded scenarios: a host that starts the JVM through the JNI invocation API without setting one, or a container framework that clears it. Logging and JDBCMetadataStore probe optional integrations through this helper, so on such a thread the SLF4J bridge or the MySQL driver would be reported as absent when it is present.

Spark's own Utils.classForName / getContextOrSparkClassLoader guard against this by falling back to a real loader instead of trusting a null context loader.

Affects Version(s)

master (1.13.0-SNAPSHOT), and earlier releases carrying kyuubi-util-scala.

Are you willing to submit PR?

Yes.