RFR: 8265765: DomainKeyStore may stop enumerating aliases if a constituting KeyStore is empty [v2]

[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.

Hai-May Chao has updated the pull request incrementally with one additional commit since the last revision:

  Testcase updated

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7386/files
  - new: https://git.openjdk.java.net/jdk/pull/7386/files/200fa1b7..a9adc254

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7386&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7386&range=00-01

  Stats: 13 lines in 1 file changed: 8 ins; 0 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