RFR: 8343437: ClassDesc.of incorrectly permitting empty names

[Chen Liang]

An oversight in the patch for [JDK-8338544](https://bugs.openjdk.org/browse/JDK-8338544) #20665 caused `ClassDesc` to incorrectly accept empty strings as binary names.  This patch fixes that problem, and unifies the checks for package and class nominal descriptors.

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Commit messages:
 - 8343437: ClassDesc.of incorrectly permitting empty names

Changes: https://git.openjdk.org/jdk/pull/21830/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21830&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343437
  Stats: 48 lines in 5 files changed: 22 ins; 6 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/21830.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21830/head:pull/21830

PR: https://git.openjdk.org/jdk/pull/21830