When reading a yaml file with 0 bytes size using `readYAML` step it returns 
LinkedList
which fails on containsKey operation:
ERROR: groovy.lang.MissingMethodException: No signature of method: java.util.LinkedList.containsKey() is applicable for argument types: (java.lang.String) values: [blabla]Possible solutions: contains(java.lang.Object), containsAll(java.util.Collection), contains(java.lang.Object), contains(java.lang.Object), containsAll([Ljava.lang.Object

Reproduce by:

                    deleteDir()
                    sh "truncate -s 0 jenkins.yml"
                    def packagesYaml = readYaml(file: "jenkins.yml")
                    echo(packagesYaml.getClass().toString())
                    echo(packagesYaml.containsKey("hello").toString())