RFR [13] 8225037: java.net.JarURLConnection::getJarEntry() throws NullPointerException

[Chris Hegarty]

The implementation of java.net.JarURLConnection has a private String,
`entryName`, that is used to hold the optional jar file entry name. If
the  JAR file URL corresponding to the JarURLConnection points to a JAR
file and not a JAR file entry, then `entryName` will be `null` ( which
should be ok ).

The default implementation of JarURLConnection::getJarEntry should only
invoke `getJarEntry(entryName)` if the name is non-null. ( This is not
an issue in the built-in JarURLConnection handler implementation, as it
already correctly handles a null `entryName` )

https://cr.openjdk.java.net/~chegar/8225037/webrev.00/

-Chris.