RFR: 8225093: Special property jdk.boot.class.path.append should not default to empty string [v3]

[Harold Seigel]

> Please review this fix for JDK-8225093 to set the default value of property jdk.boot.class.path.append to NULL instead of "".  This fix was tested by running Mach5 tiers 1-2 on Linux, Mac OS, and Windows, and Mach5 tiers 3-5 on Linux x64.  The fix was also tested by running the agent.c example in JDK-8224791.
> 
> Thanks! Harold

Harold Seigel has updated the pull request incrementally with one additional commit since the last revision:

  Add error code to test failure message

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6868/files
  - new: https://git.openjdk.java.net/jdk/pull/6868/files/a1c9218b..97d60c7e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6868&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6868&range=01-02

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6868.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6868/head:pull/6868

PR: https://git.openjdk.java.net/jdk/pull/6868