Incorrect warning when starting a job with an empty parameters set

[fmbenhassine]

After the introduction of #4910 , starting a job having an incrementer with an empty set of parameters results into an unnecessary warning:

[main] WARN org.springframework.batch.core.launch.support.TaskExecutorJobOperator -  COMMONS-LOGGING Attempting to launch job 'job' which defines an incrementer with additional parameters={{}}. Those additional parameters will be ignored.

This warning should be removed when the parameters set is empty.