RFR: 7473: Unable to view constants in constant pool page. [v2]

[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)

Alex Macdonald has updated the pull request incrementally with one additional commit since the last revision:

  spotless

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

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/348/files
  - new: https://git.openjdk.java.net/jmc/pull/348/files/80e49e19..b21b4f4c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=348&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=348&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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