S3Guard: NPE when table is already populated in dynamodb and user specifies "fs.s3a.s3guard.ddb.table.create=false"

When table is present dynamodb store and already populated, it is possible that users can specify fs.s3a.s3guard.ddb.table.create=false. In such cases, DynamoDBMetadataStore.get would end up throwing NPE as table object may not be initialized.