Fix parchment not enabled detection

Add test to cover parchment usage

common/src/main/java/net/neoforged/gradle/common/extensions/subsystems/SubsystemsExtension.java

@@ -147,7 +147,7 @@ public ParchmentExtensions(Project project) {
                                                 + ":" + DEFAULT_PARCHMENT_ARTIFACT_PREFIX + minecraftVersion
                                                 + ":" + mappingVersion
                                                 + "@zip";
-                                    })
+                                    }).orElse("")
                     )
             );
             getConflictPrefix().convention("p_");

userdev/src/functionalTest/groovy/net/neoforged/gradle/userdev/ParchmentTests.groovy

@@ -0,0 +1,59 @@
+package net.neoforged.gradle.userdev
+
+import net.neoforged.trainingwheels.gradle.functional.BuilderBasedTestSpecification
+import org.gradle.testkit.runner.TaskOutcome
+
+class ParchmentTests extends BuilderBasedTestSpecification {
+
+    @Override
+    protected void configurePluginUnderTest() {
+        pluginUnderTest = "net.neoforged.gradle.neoform";
+        injectIntoAllProject = true;
+    }
+
+    def "parchment can be used"() {
+        given:
+        def project = create("parchment_can_be_used", {
+            it.property("neogradle.subsystems.parchment.minecraftVersion", "1.21")
+            it.property("neogradle.subsystems.parchment.mappingsVersion", "2024.07.28")
+            it.build("""
+            plugins {
+                id 'net.neoforged.gradle.userdev'
+            }
+
+            java {
+                toolchain {
+                    languageVersion = JavaLanguageVersion.of(21)
+                }
+            }
+            
+            dependencies {
+                implementation 'net.neoforged:neoforge:+'
+            }
+            """)
+            it.file("src/main/java/net/neoforged/gradle/userdev/ConfigurationCacheTests.java", """
+                package net.neoforged.gradle.userdev;
+                
+                import net.minecraft.client.Minecraft;
+                
+                public class ConfigurationCacheTests {
+                    public static void main(String[] args) {
+                        System.out.println(Minecraft.getInstance().getClass().toString());
+                    }
+                }
+            """)
+            it.withToolchains()
+            it.withGlobalCacheDirectory(tempDir)
+        })
+
+        when:
+        def run = project.run {
+            it.tasks('compileJava')
+            it.arguments('--warning-mode', 'fail', '--stacktrace')
+        }
+
+        then:
+        run.task(':compileJava').outcome == TaskOutcome.SUCCESS
+        run.task(':neoFormApplyParchment').outcome == TaskOutcome.SUCCESS
+    }
+}