RFR: 8346094: Harden X509CertImpl.getExtensionValue for NPE cases [v6]

[Konanki Sreenath]

> Earlier code will trigger NPE if the certificate does not contain the extensions or if the requested extensions does not exist. The better approach for hardening **getExtensionValue** here is to to check for NULL explicitly before calling **getExtensionValue()** and avoding try-catch block which ensures the readability and maintainability.
> 
> After scanning in multiple places where invokng getExtensions on the X509CertInfo reference, the check for NULL is added in the **getKeyUsage()** as well while calling before **getExtensionValue()**
> 
> The associated tests are written and added in test class **CertificateExtensions**. Which will ensure to validate the 
> **getExtensionValue()** and **getKeyUsage()** methods in **X509CertImpl** class.

Konanki Sreenath has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23315/files
  - new: https://git.openjdk.org/jdk/pull/23315/files/dada4ce8..ddcf284a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23315&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23315&range=04-05

  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23315.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23315/head:pull/23315

PR: https://git.openjdk.org/jdk/pull/23315