RFR: JDK-8310425: [JVMCI] compiler/runtime/TestConstantDynamic: lookupConstant returned an object of incorrect type: null

[David Leopoldseder]

Fix JVMCI handling of null dynamic constants and dynamic constant errors.

For null dynamic constants the jvmci code wrongly was checking for the_null_sentinel while constantpool resolve_possibly_cached_constant_at returns nullptr for null constants.

And for errors during the bootstrap method resolve_possibly_cached_constant_at already throws the BootstrapMethodError so we can never reach the next branch.

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Commit messages:
 - 8310425: [JVMCI] fix jvmci support for null DynamicConstant and DynamicConstant in error.

Changes: https://git.openjdk.org/jdk/pull/14582/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14582&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310425
  Stats: 14 lines in 2 files changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14582.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14582/head:pull/14582

PR: https://git.openjdk.org/jdk/pull/14582