RFR: 8364751: ConstantBootstraps.explicitCast contradictory specification for null-to-primitive

[Chen Liang]

ConstantBootstraps.explicitCast behaves like a snippet of code in its specification. However, in the rest of the nominal spec, it incorrectly assumes a null `value` and a primitive `dstType` results in a ClassCastException instead of the zero value of that primitive type. This is inconsistent with that snippet and the actual code behavior.

The specification is fixed, the test for `explicitCast` is merged into the main `ConstantBootstraps` test, and a new unit test case for `value = null` and `dstType = char.class` is added, verifying the outcome is `'u0000'`.

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Commit messages:
 - Update and consolidate tests
 - 8364751: ConstantBootstraps.explicitCast violates doc contracts for null-to-int

Changes: https://git.openjdk.org/jdk/pull/26714/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26714&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364751
  Stats: 131 lines in 3 files changed: 43 ins; 79 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/26714.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26714/head:pull/26714

PR: https://git.openjdk.org/jdk/pull/26714