Fix NullAway error in DbClientAttributesExtractor

Use getDbSystem() instead of getDbSystemName() since getDbSystem() is
non-nullable and required by SemconvStability.stableDbSystemName().

Author: trask

instrumentation-api-incubator/src/main/java/io/opentelemetry/instrumentation/api/incubator/semconv/db/DbClientAttributesExtractor.java

@@ -77,14 +77,14 @@ static <REQUEST, RESPONSE> void onStartCommon(
       internalSet(
           attributes,
           DB_SYSTEM_NAME,
-          SemconvStability.stableDbSystemName(getter.getDbSystemName(request)));
+          SemconvStability.stableDbSystemName(getter.getDbSystem(request)));
       internalSet(attributes, DB_NAMESPACE, getter.getDbNamespace(request));
       internalSet(attributes, DB_QUERY_TEXT, getter.getDbQueryText(request));
       internalSet(attributes, DB_OPERATION_NAME, getter.getDbOperationName(request));
       internalSet(attributes, DB_QUERY_SUMMARY, getter.getDbQuerySummary(request));
     }
     if (SemconvStability.emitOldDatabaseSemconv()) {
-      internalSet(attributes, DB_SYSTEM, getter.getDbSystemName(request));
+      internalSet(attributes, DB_SYSTEM, getter.getDbSystem(request));
       internalSet(attributes, DB_USER, getter.getUser(request));
       internalSet(attributes, DB_NAME, getter.getDbNamespace(request));
       internalSet(attributes, DB_CONNECTION_STRING, getter.getConnectionString(request));