[jdk17u-dev] RFR: 8278826: Print error if Shenandoah flags are empty (instead of crashing)

[Aleksey Shipilev]

Clean backport to fixes Shenandoah UX problem.

Additional testing:
 - [x] Linux x86_64 fastdebug `hotspot_gc_shenandoah`
 - [x] Ad-hoc CLI tests

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Commit messages:
 - Backport 247ea71d24a251d29d9a5179e5b773df850e7261

Changes: https://git.openjdk.org/jdk17u-dev/pull/893/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=893&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278826
  Stats: 50 lines in 4 files changed: 19 ins; 21 del; 10 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/893.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/893/head:pull/893

PR: https://git.openjdk.org/jdk17u-dev/pull/893