RFR: 8277224: sun.security.pkcs.PKCS9Attributes.toString() throws NPE

[Sean Coffey]

Some elements of the PKCS9Attribute.PKCS9_OIDS array may have null value. The PKCS9Attributes.toString() and PKCS9Attributes.getAttributes() methods need to account for that.

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Commit messages:
 - 8277224: sun.security.pkcs.PKCS9Attributes.toString() throws NPE

Changes: https://git.openjdk.java.net/jdk/pull/6433/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6433&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277224
  Stats: 15 lines in 2 files changed: 10 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6433.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6433/head:pull/6433

PR: https://git.openjdk.java.net/jdk/pull/6433