patch 8.2.0031: MS-Windows: test for empty job fails

Problem:    MS-Windows: test for empty job fails
Solution:   Check for error message, make it also fail on Unix.

src/channel.c

@@ -5943,11 +5943,19 @@ job_start(
 
 	if (build_argv_from_list(l, &argv, &argc) == FAIL)
 	    goto theend;
-#ifndef USE_ARGV
+
+	// Empty command is invalid.
+#ifdef USE_ARGV
+	if (argc == 0 || *skipwhite((char_u *)argv[0]) == NUL)
+	{
+	    emsg(_(e_invarg));
+	    goto theend;
+	}
+#else
 	if (win32_build_cmd(l, &ga) == FAIL)
 	    goto theend;
 	cmd = ga.ga_data;
-	if (cmd == NULL)
+	if (cmd == NULL || *skipwhite(cmd) == NUL)
 	{
 	    emsg(_(e_invarg));
 	    goto theend;

src/testdir/test_channel.vim

@@ -1960,8 +1960,8 @@ endfunc
 
 func Test_empty_job()
   " This was crashing on MS-Windows.
-  let job = job_start([""])
-  call WaitForAssert({-> assert_equal("dead", job_status(job))})
+  call assert_fails('let job = job_start([""])', 'E474:')
+  call assert_fails('let job = job_start(["   "])', 'E474:')
 endfunc
 
 " Do this last, it stops any channel log.

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    31,
 /**/
     30,
 /**/