Fixed pdf2image conversion with imagemagick on PDFs having transparency

The target image format (jpeg) doesn't support transparency, so the
Html2ImageTest produced unusable black images when ran on a linux
machine having imagemagick package installed.

source/net/yacy/cora/util/Html2Image.java

@@ -168,7 +168,7 @@ public class Html2Image {
         try {
             // i.e. convert -density 300 -trim yacy.pdf[0] -trim -resize 1024x -crop x1024+0+0 -quality 75% yacy-convert-300.jpg
             // note: both -trim are necessary, otherwise it is trimmed only on one side. The [0] selects the first page of the pdf
-            String command = convert.getAbsolutePath() + " -density " + density + " -trim " + pdf.getAbsolutePath() + "[0] -trim -resize " + width + "x -crop x" + height + "+0+0 -quality " + quality + "% " + image.getAbsolutePath();
+            String command = convert.getAbsolutePath() + " -alpha remove -density " + density + " -trim " + pdf.getAbsolutePath() + "[0] -trim -resize " + width + "x -crop x" + height + "+0+0 -quality " + quality + "% " + image.getAbsolutePath();
             List<String> message = OS.execSynchronous(command);
             if (image.exists()) return true;
             ConcurrentLog.warn("Html2Image", "failed to create image with command: " + command);

test/java/net/yacy/cora/util/Html2ImageTest.java

@@ -26,7 +26,7 @@ public class Html2ImageTest {
     public void testPdf2image() {
         // collect pdf filenames in test directory
         File pd = new File("test/parsertest");
-        List<String> extensions = new ArrayList();
+        List<String> extensions = new ArrayList<>();
         extensions.add("pdf");
         FilenameFilter fileFilter = new ExtensionsFileFilter(extensions);
         String[] pdffiles = pd.list(fileFilter);