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)
diff --git a/src/version.c b/src/version.c
index 60aa105..3b1f963 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1710,
+/**/
     1709,
 /**/
     1708,