Make the UIService in FilePreprocessor optional

If there is no UIService: easy, we don't have to do anything in the
FilePreprocessor class.

This happens to fix a lot of stack traces show in otherwise succeeding
unit tests, reported by Jodene Jenkins.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

src/main/java/org/scijava/ui/FilePreprocessor.java

@@ -52,13 +52,14 @@
 	priority = Priority.VERY_LOW_PRIORITY + 1)
 public class FilePreprocessor extends AbstractPreprocessorPlugin {
 
-	@Parameter
+	@Parameter(required = false)
 	private UIService uiService;
 
 	// -- ModuleProcessor methods --
 
 	@Override
 	public void process(final Module module) {
+		if (uiService == null) return;
 		final ModuleItem<File> fileInput = getFileInput(module);
 		if (fileInput == null) return;