RFR: 8346094: Harden X509CertImpl.getExtensionValue for NPE cases

[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.

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Commit messages:
 - JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases
 - JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases
 - JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases
 - JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases
 - JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases
 - JDK-8346094 : Harden X509CertImpl.getExtensionValue for NPE cases

Changes: https://git.openjdk.org/jdk/pull/23315/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23315&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346094
  Stats: 229 lines in 2 files changed: 218 ins; 6 del; 5 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