Review #242102: Nashorn frames do not allow Go To Source when Java language is selected.

Please review the fix of bug
https://netbeans.org/bugzilla/show_bug.cgi?id=242102

Description:
Consider a program, that contains a mix of Java and JavaScript code. 
When execution is suspended in a Java code (session language is Java), 
then Go To Source action is not enabled on JavaScript stack frames in 
Debugging view. This is because JavaScript frames have no source URL for 
Java language.

Solution:
When source URL for the current language is null, iterate through 
available languages and test their URL for null. Enable Go To Source 
action when there is at least one non-null URL.

Impact assesment:
Debugging in Nashorn JavaScript code is affected.

Risk:
Low, we only make a JavaScript-related functionality available in a 
situation in which it would normally be inaccessible.

The bugfix was reviewed.

http://hg.netbeans.org/main-silver/rev/11791f0c9595

Thanks,
Martin