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_getvar.vim b/src/testdir/test_getvar.vim
index 5da5991..7ced44d 100644
--- a/src/testdir/test_getvar.vim
+++ b/src/testdir/test_getvar.vim
@@ -142,6 +142,11 @@
   call assert_equal({'func has': 'no dict'}, get(l:F, 'dict', {'func has': 'no dict'}))
   call assert_equal(0, get(l:F, 'dict'))
   call assert_equal([], get(l:F, 'args'))
+  let NF = test_null_function()
+  call assert_equal('', get(NF, 'name'))
+  call assert_equal(NF, get(NF, 'func'))
+  call assert_equal(0, get(NF, 'dict'))
+  call assert_equal([], get(NF, 'args'))
 endfunc
 
 " get({partial}, {what} [, {default}]) - in test_partial.vim