RFR: 8265765: DomainKeyStore may stop enumerating aliases if a constituting KeyStore is empty

[Hai-May Chao]

This is to fix `DomainKeyStore::engineAliases` to take into account that there may be empty keystore(s) within the collection of keystores of a domain keystore.

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Commit messages:
 - 8265765: DomainKeyStore may stop enumerating aliases if a constituting KeyStore is empty

Changes: https://git.openjdk.java.net/jdk/pull/7386/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7386&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265765
  Stats: 128 lines in 2 files changed: 121 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7386.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7386/head:pull/7386

PR: https://git.openjdk.java.net/jdk/pull/7386