RFR: 8266400: importkeystore fails to a password less pkcs12 keystore

[Hai-May Chao]

Please review the fix to address keytool -importkeystore failure when importing to a password-less PKCS12 keystore.

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Commit messages:
 - 8266400: importkeystore fails to a password less pkcs12 keystore

Changes: https://git.openjdk.java.net/jdk/pull/4119/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4119&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266400
  Stats: 92 lines in 2 files changed: 89 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4119.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4119/head:pull/4119

PR: https://git.openjdk.java.net/jdk/pull/4119