Review request: JDK-8067366 Allow java.{endorsed,ext}.dirs property be set to empty string

[Mandy Chung]

Webrev:
http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8067366/webrev.00/

There is no harm to support java.{endorsed,ext}.dirs property be set to 
an empty string as the value.  This patch will allow setting 
-Djava.{endorsed,ext}.dirs="" which will make it easier for some 
applications such as IntelliJ to migrate.  Setting such property to an 
empty string has the same runtime behavior as the endorsed or extension 
mechanism is not supported in JDK 9.

Mandy