RFR: 7076: Rules and Analysis code has exceptions during uitests

[Henrik Dafgård]

Fixes exceptions encountered during uitests due to empty grouping arrays.

-------------

Commit messages:
 - Fix exceptions thrown in allocation rules

Changes: https://git.openjdk.java.net/jmc/pull/189/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=189&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7076
  Stats: 34 lines in 2 files changed: 6 ins; 0 del; 28 mod
  Patch: https://git.openjdk.java.net/jmc/pull/189.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/189/head:pull/189

PR: https://git.openjdk.java.net/jmc/pull/189