[15] RFR: 8240626: Some of the java.time.chrono.Eras return empty display name for some styles and locales

Hello, Please review the fix to the following issue: https://bugs.openjdk.java.net/browse/JDK-8240626 The proposed chageset is located at: https://cr.openjdk.java.net/~naoto/8240626/webrev.00/ In some locales, CLDR only provides partial translations of era names, e.g., only HEISEI and REIWA are provided for Japanese Calendar in Greek locale. CLDRConverter needs to supplement those missing translations from parent locales. Naoto