Allow for optional tagger and message in Tag

We should be more lenient when tagging without an
tagger or message. Currently, we will throw an NPE
which is incorrect behavior.

Change-Id: I04e30ce25a9432e4ca56c3f29658ecb24fb18d24
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectInserter.java

 			w.write(tag.getTag());
 			w.write("\n");
 
-			w.write("tagger ");
-			w.write(tag.getTagger().toExternalString());
-			w.write('\n');
+			if (tag.getTagger() != null) {
+				w.write("tagger ");
+				w.write(tag.getTagger().toExternalString());
+				w.write('\n');
+			}
 
 			w.write('\n');
-			w.write(tag.getMessage());
+			if (tag.getMessage() != null)
+				w.write(tag.getMessage());
 			w.close();
 		} catch (IOException err) {
 			// This should never occur, the only way to get it above is