FOR REVIEW: CODETOOLS-7900902 Agent.Task.getResourceData incorrect for empty files

[Brian Kurotsuchi]

https://bugs.openjdk.java.net/browse/CODETOOLS-7900902

Diffs:
http://cr.openjdk.java.net/~bkurotsu/7900902/

Original contribution by Alexey Fedorov, modified for use by me.

Summary:
Javadoc says that (for the downstream method which uses the wire 
protocol result):
     RETURNS - An input stream for reading the resource, or null if the 
resource could not be found

We currently return a null (missing) case for both missing and empty 
results to the resource request.  JT Agent wire protocol doesn't support 
missing vs zero length.  This code modification makes (missing == -1), 
(zero length == 0).

Alexey has already positively tested the fix for his needs.