patch 8.2.0032: MS-Windows: test for blank job fails

Problem:    MS-Windows: test for blank job fails
Solution:   Check before escaping.
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index a7c4009..4dac7aa 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1962,6 +1962,8 @@
   " This was crashing on MS-Windows.
   call assert_fails('let job = job_start([""])', 'E474:')
   call assert_fails('let job = job_start(["   "])', 'E474:')
+  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.