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/globals.h

@@ -1699,7 +1699,7 @@ EXTERN char e_invalblob[]	INIT(= N_("E978: Invalid operation for Blob"));
 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"));

src/testdir/test_vim9_expr.vim

@@ -1917,6 +1917,7 @@ def Test_expr7_dict()
   CheckDefExecFailure(['var x: dict<string> = #{a: "x", b: 134}'], 'E1012:', 1)
 
   CheckDefFailure(['var x = ({'], 'E723:', 2)
+  CheckDefExecFailure(['{}[getftype("")]'], 'E716: Key not present in Dictionary: ""', 1)
 enddef
 
 def Test_expr7_dict_vim9script()

src/version.c

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

src/vim9execute.c

@@ -2607,6 +2607,8 @@ call_def_function(
 		    tv = STACK_TV_BOT(-1);
 		    // no need to check for VAR_STRING, 2STRING will check.
 		    key = tv->vval.v_string;
+		    if (key == NULL)
+			key = (char_u *)"";
 
 		    if ((di = dict_find(dict, key, -1)) == NULL)
 		    {