FINERACT-2630: Use Collection.isEmpty() instead of size() comparison in fineract-savings

[heissenberg06]

Description

Replaces 6 occurrences of Collection.size() > 0 emptiness checks with !Collection.isEmpty() across the fineract-savings module (SavingsAccount, SavingsAccountRepositoryWrapper, SavingsSchedularInterestPoster), as flagged by SonarCloud rule java:S1155 ("Collection.isEmpty() should be used to test for emptiness"). This mirrors the cleanup done for the fineract-loan module in FINERACT-2627.

The change is behavior-preserving — !collection.isEmpty() is equivalent to collection.size() > 0 — and is a readability/consistency cleanup only. No API, schema, or functional behavior changes. JsonArray checks and index-bound size() usages were intentionally left untouched. Covered by existing tests.

JIRA: https://issues.apache.org/jira/browse/FINERACT-2630

Checklist

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[adamsaghy]

LGTM