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/globals.h b/src/globals.h
index 5bdbad6..379a532 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1699,7 +1699,7 @@
 EXTERN char e_toomanyarg[]	INIT(= N_("E118: Too many arguments for function: %s"));
 EXTERN char e_toofewarg[]	INIT(= N_("E119: Not enough arguments for function: %s"));
 EXTERN char e_func_deleted[]	INIT(= N_("E933: Function was deleted: %s"));
-EXTERN char e_dictkey[]		INIT(= N_("E716: Key not present in Dictionary: %s"));
+EXTERN char e_dictkey[]		INIT(= N_("E716: Key not present in Dictionary: \"%s\""));
 EXTERN char e_listreq[]		INIT(= N_("E714: List required"));
 EXTERN char e_listblobreq[]	INIT(= N_("E897: List or Blob required"));
 EXTERN char e_list_end[]	INIT(= N_("E697: Missing end of List ']': %s"));