@Override
    public Serde<?> keySerde() {
        return config.defaultKeySerde();
    }
Create a {@link MockProcessorContext} with a specified taskId and null stateDir.

@param config   a {@link Properties} object, used to configure the context and the processor.
@param taskId   a {@link TaskId}, which the context makes available via {@link MockProcessorContext#taskId()}.
@param stateDir a {@link File}, which the context makes available viw {@link MockProcessorContext#stateDir()}.
streams/test-utils/src/main/java/org/apache/kafka/streams/processor/MockProcessorContext.java