I have a trivial jobs that uses polling for a git repository hosted under gerrit.
The job status page shows that the job was triggered by an SCM change but 'Changes' are empty.

I use the script console to look at culprits or changeset and I get an empty list.

For example:
{{import hudson.*

jobName = 'myjob'
myJob = Jenkins.instance.getItemByFullName(jobName)
lastBuild = myJob.getBuildByNumber(1)
cset = lastBuild.getChangeSet()
println "cset = $cset"
println "cset.getItems() = ${cset.getItems()}"
culprits = lastBuild.getCulprits()
println "culprits =$culprits"
null

cset = hudson.plugins.git.GitChangeSetList@7aca95e7
cset.getItems() = []
culprits =[]
}}