RFR: 8364751: ConstantBootstraps.explicitCast contradictory specification for null-to-primitive [v4]

[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'`.

Chen Liang has updated the pull request incrementally with one additional commit since the last revision:

  Update src/java.base/share/classes/java/lang/invoke/ConstantBootstraps.java
  
  Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26714/files
  - new: https://git.openjdk.org/jdk/pull/26714/files/e00370b9..631227b2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26714&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26714&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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