Fix issues with declaring clean run configs

Author: Jonathing

fg-plugin/src/main/groovy/net/minecraftforge/gradle/SlimeLauncherExec.groovy

@@ -22,6 +22,7 @@ import org.gradle.api.tasks.Input
 import org.gradle.api.tasks.InputDirectory
 import org.gradle.api.tasks.InputFile
 import org.gradle.api.tasks.JavaExec
+import org.gradle.api.tasks.Optional
 import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.TaskProvider
 import org.gradle.language.base.plugins.LifecycleBasePlugin
@@ -112,7 +113,7 @@ import java.nio.file.Files
                 '--metadata', this.metadataZip.get().asFile.absolutePath,
                 '--')
 
-            this.args(this.mcBootstrapArgs.get().toArray())
+            this.args(this.mcBootstrapArgs.getOrElse(List.of()).toArray())
         }
 
         Files.createDirectories(this.workingDir.toPath())
@@ -125,6 +126,6 @@ import java.nio.file.Files
     /** The location of the {@code metadata.zip} artifact produced by the Minecraft Mavenizer. */
     abstract @InputFile RegularFileProperty getMetadataZip()
     abstract @Input Property<String> getBootstrapMainClass()
-    abstract @Input ListProperty<String> getMcBootstrapArgs()
-    abstract @Input Property<Boolean> getClient()
+    abstract @Input @Optional ListProperty<String> getMcBootstrapArgs()
+    abstract @Input @Optional Property<Boolean> getClient()
 }

fg-plugin/src/main/groovy/net/minecraftforge/gradle/SlimeLauncherOptions.java

@@ -616,7 +616,7 @@ static NamedDomainObjectContainer<SlimeLauncherOptions> container(ObjectFactory
     /// @param task The task to apply the options to
     final void apply(SlimeLauncherExec task) {
         if (this.mainClass.map(Util::nullIfEmpty).isPresent())
-            task.getMainClass().set(this.mainClass);
+            task.getBootstrapMainClass().set(this.mainClass);
 
         if (this.args.map(Util::nullIfEmpty).isPresent())
             task.getMcBootstrapArgs().set(this.args);