Fix #27346: When The FONT object is blank, the dimensions are calculated wong.

This commit fixes boundbox for empty curve-typed objects. Because of strange
reason (-1,-1,-1) and (1,1,1) was used as boundbox. Now it uses zero-sized
boundbox (as it's done for meshes).

This commit makes nothig with that single space character text from the report.

source/blender/blenkernel/intern/displist.c

@@ -1973,8 +1973,9 @@ static void boundbox_displist(Object *ob)
 		}
 		
 		if(!doit) {
-			min[0] = min[1] = min[2] = -1.0f;
-			max[0] = max[1] = max[2] = 1.0f;
+			/* there's no geometry in displist, use zero-sized boundbox */
+			zero_v3(min);
+			zero_v3(max);
 		}
 		
 	}