[Rev 02] RFR: 6724: Agent argument as a path to a probe definition XML should be optional

[Kangcheng Xu]

> This patch addresses [JMC-6724: Agent argument as a path to a probe definition XML should be
> optional](https://bugs.openjdk.java.net/browse/JMC-6724). It makes the agent argument optional and probes can always be
> defined later via JMX. If such an argument is missing, an empty registry is created with no transform descriptor.

Kangcheng Xu has refreshed the contents of this pull request, and previous commits have been removed. The incremental
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  Update documentation

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/63/files
  - new: https://git.openjdk.java.net/jmc/pull/63/files/b7e05c34..daf4bf19

Webrevs:
 - full: https://webrevs.openjdk.java.net/jmc/63/webrev.02
 - incr: https://webrevs.openjdk.java.net/jmc/63/webrev.01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/63.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/63/head:pull/63

PR: https://git.openjdk.java.net/jmc/pull/63