patch 8.2.0299: Vim9: ISN_STORE with argument not tested

Problem:    Vim9: ISN_STORE with argument not tested.  Some cases in tv2bool()
            not tested.
Solution:   Add tests.  Add test_unknown() and test_void().
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 312d633..a5b6e44 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -765,6 +765,23 @@
   assert_equal(false, ![2])
   assert_equal(true, !!'asdf')
   assert_equal(true, !![2])
+
+  assert_equal(true, !test_null_partial())
+  assert_equal(false, !{-> 'yes'})
+
+  assert_equal(true, !test_null_dict())
+  assert_equal(true, !{})
+  assert_equal(false, !{'yes': 'no'})
+
+  assert_equal(true, !test_null_job())
+  assert_equal(true, !test_null_channel())
+
+  assert_equal(true, !test_null_blob())
+  assert_equal(true, !0z)
+  assert_equal(false, !0z01)
+
+  assert_equal(true, !test_void())
+  assert_equal(true, !test_unknown())
 enddef
 
 func Test_expr7_fails()