Try adding defaults

vars/buildStep.groovy

@@ -1,9 +1,19 @@
 #!/usr/bin/env groovy
 
+import groovy.transform.Field
+
+@Field
+Map defaults = [
+    buildCmd: 'make all'
+]
+
+
 def call(Map config = [:]) {
-    echo "Hello, ${config.name}"
+    assert config.artifacts : 'must specify artifacts!'
-    if ( fileExists('Makefile')) {
+
+    config = defaults + config
+    if ( fileExists(config.buildCmd)) {
         sh 'make'
     }
-    echo 'Done'
+
 }