Fix false error with optional param (#6811)

Signed-off-by: Ben Sherman <bentshermann@gmail.com>

Author: bentsherman

modules/nextflow/src/test/groovy/nextflow/script/parser/v2/ScriptLoaderV2Test.groovy

@@ -295,6 +295,30 @@ class ScriptLoaderV2Test extends Dsl2Spec {
         outputs.getFiles().size() == 3
     }
 
+    def 'should allow optional param' () {
+
+        given:
+        def session = new Session()
+        def parser = new ScriptLoaderV2(session)
+
+        def TEXT = '''
+            params {
+                path: Path?
+            }
+
+            workflow {
+                params.path
+            }
+            '''
+
+        when:
+        parser.parse(TEXT)
+        parser.runScript()
+
+        then:
+        parser.getResult() == null
+    }
+
     def 'should support enums' () {
 
         given:

modules/nf-lang/src/main/java/nextflow/script/control/ScriptToGroovyVisitor.java

@@ -150,7 +150,7 @@ public void visitParams(ParamBlockNode node) {
             .map((param) -> {
                 var name = constX(param.getName());
                 var type = classX(param.getType());
-                var optional = constX(type.getNodeMetaData(ASTNodeMarker.NULLABLE) != null);
+                var optional = constX(param.getType().getNodeMetaData(ASTNodeMarker.NULLABLE) != null);
                 var arguments = param.hasInitialExpression()
                     ? args(name, type, optional, param.getInitialExpression())
                     : args(name, type, optional);