URLClassLoader does not specify behavior when URL array contains null

Some jdi tests were broken by
8198484: URLClassPath should use an ArrayDeque instead of a Stack
Affected tier1 Tests:
com/sun/jdi/EarlyReturnNegativeTest.java
com/sun/jdi/EarlyReturnTest.java


----------System.out:(17/926)----------
vmOpts: '-XX:MaxRAMPercentage=8 -ea -esa -Xmx512m'
javaOpts: ''
JVM version:11-internal
JDI version: 9.0
JVM description: Java Debug Interface (Reference Implementation) version 9.0
Java Debug Wire Protocol (Reference Implementation) version 9.0
JVM Debug Interface version 9.0
JVM version 11-internal (Java HotSpot(TM) 64-Bit Server VM, mixed mode, aot, sharing)
Exception in thread "main" java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException
at java.base/java.util.ArrayDeque.addLast(ArrayDeque.java:304)
at java.base/java.util.ArrayDeque.add(ArrayDeque.java:491)
at java.base/jdk.internal.loader.URLClassPath.<init>(URLClassPath.java:145)
at java.base/jdk.internal.loader.URLClassPath.<init>(URLClassPath.java:162)
at java.base/java.net.URLClassLoader.<init>(URLClassLoader.java:163)
at EarlyReturnTarg.<init>(EarlyReturnTest.java:88)
at EarlyReturnTarg.<clinit>(EarlyReturnTest.java:96)
----------System.err:(22/1256)----------
[1ms] run args: [EarlyReturnTarg]
[907ms] Note: Unexpected Debuggee Exception: java.lang.NullPointerException at line 304
[937ms] detailMessage = null
java.lang.RuntimeException: VM Disconnected before requested event occurred
at TestScaffold.waitForRequestedEvent(TestScaffold.java:742)
at TestScaffold.resumeTo(TestScaffold.java:791)
at TestScaffold.resumeTo(TestScaffold.java:844)
at TestScaffold.startTo(TestScaffold.java:377)
at TestScaffold.startToMain(TestScaffold.java:369)
at EarlyReturnTest.runTests(EarlyReturnTest.java:710)
at TestScaffold.startTests(TestScaffold.java:431)
at EarlyReturnTest.main(EarlyReturnTest.java:312)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:564)
at com.sun.javatest.regtest.agent.MainActionHelper$SameVMRunnable.run(MainActionHelper.java:229)
at java.base/java.lang.Thread.run(Thread.java:841)

JavaTest Message: Test threw exception: java.lang.RuntimeException