Gradle run task only on defined property

I am struggling to let a task only execute when a specific value is defined.

I'm using Gradle 3.5.

task signJar(type: SignJar, dependsOn: reobfJar) {
    onlyIf {
        project.hasProperty('mod_keystore')
    }

    keyStore = project.keyStore
    alias = project.keyStoreAlias
    storePass = project.keyStorePass
    keyPass = project.keyStoreKeyPass
    inputFile = jar.archivePath
    outputFile = jar.archivePath
}

As you can see, I already tried the onlyIf statement, but the task still runs. This results into a crash:

Could not get unknown property 'keyStore' for root project 'JustAnotherEnergy' of type org.gradle.api.Project.

The property 'mod_keystore' is no where defined, but the code get's executed.

task signJar(type: SignJar, dependsOn: reobfJar) {
    if(project.hasProperty('mod_keystore')) {
        keyStore = project.keyStore
        alias = project.keyStoreAlias
        storePass = project.keyStorePass
        keyPass = project.keyStoreKeyPass
        inputFile = jar.archivePath
        outputFile = jar.archivePath
    }
}

This works. The code does not get executed, but I'm running into other problems: If the property 'mod_keystore' is not defined, Gradle can't set a value for the for example 'keyStore' property, but the task SignJar requires this values to be set.

This means this task should only be executed when the property 'mod_keystore' is defined. If it is not defined, the task should be skipped.