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.

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Commit messages:
 - Make the agent argument optional and create an empty registry if missing

Changes: https://git.openjdk.java.net/jmc/pull/63/files
 Webrev: https://webrevs.openjdk.java.net/jmc/63/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6724
  Stats: 20 lines in 3 files changed: 17 ins; 1 del; 2 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