patch 8.2.1710: Vim9: list of list type can be wrong

Problem:    Vim9: list of list type can be wrong.
Solution:   Use VAR_UNKNOWN for empty list.  Recognize VAR_UNKNOWN when
            looking for a common type. (closes #6979)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1710,
 /**/
     1709,
 /**/