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

[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

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Commit messages:
 - 8225093: Special property jdk.boot.class.path.append should not default to empty string

Changes: https://git.openjdk.java.net/jdk/pull/6868/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6868&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225093
  Stats: 116 lines in 4 files changed: 114 ins; 0 del; 2 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