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/if_lua.c b/src/if_lua.c
index 6e33734..9400e3a 100644
--- a/src/if_lua.c
+++ b/src/if_lua.c
@@ -562,6 +562,10 @@
 	    tv->v_type = VAR_SPECIAL;
 	    tv->vval.v_number = (varnumber_T) lua_toboolean(L, pos);
 	    break;
+	case LUA_TNIL:
+	    tv->v_type = VAR_SPECIAL;
+	    tv->vval.v_number = VVAL_NULL;
+	    break;
 	case LUA_TSTRING:
 	    tv->v_type = VAR_STRING;
 	    tv->vval.v_string = vim_strsave((char_u *) lua_tostring(L, pos));