Correct single file reference since libraries can now optionally exist such as...

Correct single file reference since libraries can now optionally exist such as when there are no sources.

buildSrc/src/main/java/cc/squirreljme/plugin/util/SingleTaskOutputFile.java

@@ -12,6 +12,7 @@ package cc.squirreljme.plugin.util;
 import java.nio.file.Path;
 import java.util.concurrent.Callable;
 import org.gradle.api.Task;
+import org.gradle.api.file.FileCollection;
 
 /**
  * This takes the output of a task and provides a {@link Callable} so that
@@ -48,6 +49,11 @@ public class SingleTaskOutputFile
 	@Override
 	public Path call()
 	{
-		return this.task.getOutputs().getFiles().getSingleFile().toPath();
+		FileCollection files = this.task.getOutputs().getFiles();
+		
+		if (files.getFiles().isEmpty())
+			return null;
+		
+		return files.getSingleFile().toPath();
 	}
 }