patch 8.2.1611: Vim9: cannot pass "true" to nr2char()

Problem:    Vim9: cannot pass "true" to nr2char().
Solution:   use tv_get_bool_chk(). (closes #6878)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 84fc7c9..b3b6b32 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1529,6 +1529,10 @@
   assert_equal(s, list2str(l, true))
 enddef
 
+def Test_nr2char()
+  assert_equal('a', nr2char(97, true))
+enddef
+
 def Fibonacci(n: number): number
   if n < 2
     return n