patch 8.2.0501: Vim9: script test fails when channel feature is missing

Problem:    Vim9: script test fails when channel feature is missing.
Solution:   Add a has() condition.

src/testdir/test_vim9_script.vim

@@ -164,11 +164,13 @@ def Test_assignment()
   let thedict: dict<any>
   assert_equal({}, thedict)
 
+  if has('channel')
     let thejob: job
     assert_equal(test_null_job(), thejob)
 
     let thechannel: channel
     assert_equal(test_null_channel(), thechannel)
+  endif
 enddef
 
 func Test_assignment_failure()

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    501,
 /**/
     500,
 /**/