patch 8.1.0672: the Lua interface doesn't know about v:null

Problem:    The Lua interface doesn't know about v:null.
Solution:   Add Lua support for v:null. (Uji, closes #3744)
diff --git a/src/version.c b/src/version.c
index 3de4461..623d2a0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    672,
+/**/
     671,
 /**/
     670,