patch 8.0.0447: getting font name does not work on X11

Problem:    Getting font name does not work on X11.
Solution:   Implement gui_mch_get_fontname() for X11.  Add more GUI tests.
            (Kazunobu Kuriyama)
diff --git a/src/syntax.c b/src/syntax.c
index 11f679c..b13bd93 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -8169,7 +8169,7 @@
 	    || HL_TABLE()[idx].sg_gui_fg_name != NULL
 	    || HL_TABLE()[idx].sg_gui_bg_name != NULL
 	    || HL_TABLE()[idx].sg_gui_sp_name != NULL
-	    || HL_TABLE()[idx].sg_font_name != NUL
+	    || HL_TABLE()[idx].sg_font_name != NULL
 #endif
 	    || (check_link && (HL_TABLE()[idx].sg_set & SG_LINK)));
 }