patch 8.2.1854: Vim9: crash when throwing exception for NULL string

Problem:    Vim9: crash when throwing exception for NULL string. (Dhiraj
            Mishra)
Solution:   Handle NULL string like empty string. (closes #7139)

src/errors.h

@@ -280,4 +280,6 @@ EXTERN char e_missing_name_after_dot[]
 	INIT(= N_("E1127: Missing name after dot"));
 EXTERN char e_endblock_without_block[]
 	INIT(= N_("E1128: } without {"));
+EXTERN char e_throw_with_empty_string[]
+	INIT(= N_("E1129: Throw with empty string"));
 #endif

src/testdir/test_vim9_script.vim

@@ -617,6 +617,21 @@ def Test_throw_vimscript()
       endtry
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+    vim9script
+    def Func()
+      throw @r
+    enddef
+    var result = ''
+    try
+      Func()
+    catch /E1129:/
+      result = 'caught'
+    endtry
+    assert_equal('caught', result)
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def Test_error_in_nested_function()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1854,
 /**/
     1853,
 /**/

src/vim9execute.c

@@ -1381,6 +1381,8 @@ call_def_function(
 		tv = STACK_TV_BOT(0);
 		tv->v_type = VAR_STRING;
 		tv->v_lock = 0;
+		// This may result in NULL, which should be equivalent to an
+		// empty string.
 		tv->vval.v_string = get_reg_contents(
 					  iptr->isn_arg.number, GREG_EXPR_SRC);
 		++ectx.ec_stack.ga_len;
@@ -2082,6 +2084,13 @@ call_def_function(
 	    case ISN_THROW:
 		--ectx.ec_stack.ga_len;
 		tv = STACK_TV_BOT(0);
+		if (tv->vval.v_string == NULL
+				       || *skipwhite(tv->vval.v_string) == NUL)
+		{
+		    emsg(_(e_throw_with_empty_string));
+		    goto failed;
+		}
+
 		if (throw_exception(tv->vval.v_string, ET_USER, NULL) == FAIL)
 		{
 		    vim_free(tv->vval.v_string);