RFR: 8270946: X509CertImpl.getFingerprint should not return the empty String [v2]

[Sean Mullan]

> Please review this fix to change the internal `X509CertImpl.getFingerprint` method to not return "" as a fingerprint if there is an error generating that fingerprint. Instead, `null` is now returned, and "" is no longer cached as a valid fingerprint. Although errors generating fingerprints should be very rare, this is a cleaner way to handle them.
> 
> Also, debugging messages have been added when there is an exception. And, as a memory/performance improvement, `X509CertImpl.getFingerprint` now calls `X509CertImpl.getEncodedInternal` which avoids cloning the encoded bytes if the `Certificate` is an instance of `X509CertImpl`.

Sean Mullan has updated the pull request incrementally with one additional commit since the last revision:

  Refactor X509CertImpl.getFingerprint(String, Debug).
  Treat null fingerprint as untrusted.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4891/files
  - new: https://git.openjdk.java.net/jdk/pull/4891/files/50b47ad9..d8652367

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4891&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4891&range=00-01

  Stats: 35 lines in 4 files changed: 10 ins; 3 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4891.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4891/head:pull/4891

PR: https://git.openjdk.java.net/jdk/pull/4891