RFR: 7473: Unable to view constants in constant pool page.

[Alex Macdonald]

This PR addresses JMC-7473 [[0]](https://bugs.openjdk.java.net/browse/JMC-7473), in which the Constant Pools page throws an exception and does not display. The issue here is in ParserStats, and happens if the stacktrace object doesn't have any frames - but there is a retrieval for the first frame. 

Before:
![2022-01-10-105031_790x151_scrot](https://user-images.githubusercontent.com/10425301/148796099-b32c4802-7b12-4489-ab09-0aa65a73cea9.png)

After:
![2022-01-10-105044_668x458_scrot](https://user-images.githubusercontent.com/10425301/148796118-d066f332-d0a3-4982-9502-b26efd2ef01b.png)

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Commit messages:
 - 7473: Unable to view constants in constant pool page

Changes: https://git.openjdk.java.net/jmc/pull/348/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=348&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7473
  Stats: 9 lines in 1 file changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jmc/pull/348.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/348/head:pull/348

PR: https://git.openjdk.java.net/jmc/pull/348