chore: deprecate getMasterKeyIds()

Author: imabhichow

src/main/java/com/amazonaws/encryptionsdk/CryptoResult.java

@@ -69,6 +69,7 @@ public List<K> getMasterKeys() {
   }
 
   /** Convenience method for retrieving the keyIds in the results from {@link #getMasterKeys()}. */
+  @Deprecated
   public List<String> getMasterKeyIds() {
     final List<String> result = new ArrayList<>(masterKeys_.size());
     for (final MasterKey<K> mk : masterKeys_) {

src/main/java/com/amazonaws/encryptionsdk/internal/DecryptionHandler.java

@@ -890,7 +890,9 @@ public CiphertextHeaders getHeaders() {
 
   @Override
   public List<K> getMasterKeys() {
-    return Collections.singletonList(dataKey_.getMasterKey());
+    return dataKey_.getMasterKey() == null
+        ? Collections.emptyList()
+        : Collections.singletonList(dataKey_.getMasterKey());
   }
 
   @Override