patch 8.2.1924: Vim9: crash when indexing dict with NULL key

Problem:    Vim9: crash when indexing dict with NULL key.
Solution:   Use empty string instead of NULL. (closes #7229)  Make error
            message more useful for empty string.
diff --git a/src/version.c b/src/version.c
index 52d8683..0da7888 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1924,
+/**/
     1923,
 /**/
     1922,