patch 8.2.0300: Vim9: expression test fails without channel support

Problem:    Vim9: expression test fails without channel support.
Solution:   Add has('channel') check.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index a5b6e44..fb7d39f 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -773,8 +773,10 @@
   assert_equal(true, !{})
   assert_equal(false, !{'yes': 'no'})
 
-  assert_equal(true, !test_null_job())
-  assert_equal(true, !test_null_channel())
+  if has('channel')
+    assert_equal(true, !test_null_job())
+    assert_equal(true, !test_null_channel())
+  endif
 
   assert_equal(true, !test_null_blob())
   assert_equal(true, !0z)