PushCertificate: Omit null pushee from text representation

Change-Id: Ie9546f2e0e9ee62e0a3c919572153b6076355195

org.eclipse.jgit.test/tst/org/eclipse/jgit/transport/PushCertificateParserTest.java

@@ -360,6 +360,7 @@ public class PushCertificateParserTest {
 		PushCertificate cert = parser.build();
 		assertEquals("0.1", cert.getVersion());
 		assertNull(cert.getPushee());
+		assertFalse(cert.toText().contains(PushCertificateParser.PUSHEE));
 	}
 
 	private static String concatPacketLines(String input, int begin, int end)

org.eclipse.jgit/src/org/eclipse/jgit/transport/PushCertificate.java

@@ -219,9 +219,11 @@ public class PushCertificate {
 		StringBuilder sb = new StringBuilder()
 				.append(VERSION).append(' ').append(version).append('\n')
 				.append(PUSHER).append(' ').append(getPusher())
-				.append('\n')
-				.append(PUSHEE).append(' ').append(pushee).append('\n')
-				.append(NONCE).append(' ').append(nonce).append('\n')
+				.append('\n');
+		if (pushee != null) {
+			sb.append(PUSHEE).append(' ').append(pushee).append('\n');
+		}
+		sb.append(NONCE).append(' ').append(nonce).append('\n')
 				.append('\n');
 		for (ReceiveCommand cmd : commands) {
 			sb.append(cmd.getOldId().name())