patch 8.2.0634: crash with null partial and blob

Problem:    Crash with null partial and blob.
Solution:   Check for NULL pointer.  Add more tests. (Yegappan Lakshmanan,
            closes #5984)
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index cd9611c..28350bb 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -41,6 +41,9 @@
   call assert_fails('call mkdir("Xfile", "p")', 'E739')
   call delete('Xfile')
   call delete('Xmkdir', 'rf')
+  call assert_equal(0, mkdir(test_null_string()))
+  call assert_fails('call mkdir([])', 'E730')
+  call assert_fails('call mkdir("abc", [], [])', 'E745')
 endfunc
 
 func Test_line_continuation()
@@ -223,6 +226,7 @@
   call assert_fails('execute test_null_blob()', 'E976:')
   execute test_null_string()
   call assert_fails('execute test_null_partial()', 'E729:')
+  call assert_fails('execute test_unknown()', 'E908:')
   if has('job')
     call assert_fails('execute test_null_job()', 'E908:')
     call assert_fails('execute test_null_channel()', 'E908:')