Empty destinationDir as other task input

Help/Discuss
1

I would like to merge the coverage of many of my included project of a composite build. But only some of them have a coverage.

1st, I create task for all of my included project in order to copy the potential jacoco exec file
Something like

gradle.includedBuilds.each { p ->
    tasks.register("copyCoverage${p.name}", Copy, {
        from ("${p.projectDir.path}") {
            include "**/*.exec"
            exclude '**/jacocoMerge.exec'
            includeEmptyDirs = false
        }
        into coverageDir + File.separator + p.name
    })
}

Then, I wrote a task for merge all of them

task jacocoMerge(type: JacocoMerge, group: 'Coverage reports') {
    group = 'CDR'
    dependsOn copyCoverage
    description = 'Merge all jacoco execution data from all java sub-projects into on execution file.'
    gradle.includedBuilds.each { p ->
        	executionData (tasks."copyCoverage${p.name}".destinationDir)
    }
}

But, as some of my project doesn’t have any exec file (=no output files for some of my copyCoverage{}), the jacocoMerge task failed with “Unable to read [… one of the destinationDir …]”

If I try to protect it by

gradle.includedBuilds.each { p ->
    def f = tasks."copyCoverage${p.name}".destinationDir
    if(f.exists()) {
        println "copy from ${f.absolutePath}"
        executionData (tasks."copyCoverage${p.name}".destinationDir)
    }
}

I think I disable the lazy configuration and I have “No value has been specified for property ‘executionData’.”

How should I handle this issue ?