[11] RFR JDK-8191170: Clarify if java.class.path can be undefined

[mandy chung]

http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8191170/webrev.00/index.html
A small adjustment in the javadoc.  It clarifies what an empty value of 
"java.class.path" property is interpreted by the built-in application 
class loader.

Mandy