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.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1924,
 /**/
     1923,
 /**/