eval/encode: Make sure that encoder can encode NULL variables

Adds two undocumented v: variables: _null_list and _null_dict because I do not 
know a reproducible way to get such lists (though I think I heard about this) 
and dictionaries (do not remember hearing about them). NULL strings are obtained 
using $XXX_UNEXISTENT_VAR_XXX.

Fixes crash in json_encode($XXX_UNEXISTENT_VAR_XXX). Other added tests worked 
fine before this commit.

1 files changed, 12 insertions, 0 deletions

diff --git a/test/functional/eval/json_functions_spec.lua b/test/functional/eval/json_functions_spec.lua
index b788c2cf07..0a7c4cf669 100644
--- a/test/functional/eval/json_functions_spec.lua
+++ b/test/functional/eval/json_functions_spec.lua
@@ -784,4 +784,16 @@ describe('json_encode() function', function()
     eq([["\u0000\u0001\u0002\u0003\u0004\u0005\u0006\u0007\b\t\n\u000B\f\r\u000E\u000F\u0010\u0011\u0012\u0013"]],
        eval('json_encode({"_TYPE": v:msgpack_types.string, "_VAL": ["\n\1\2\3\4\5\6\7\8\9", "\11\12\13\14\15\16\17\18\19"]})'))
   end)
+
+  it('can dump NULL string', function()
+    eq('""', eval('json_encode($XXX_UNEXISTENT_VAR_XXX)'))
+  end)
+
+  it('can dump NULL list', function()
+    eq('[]', eval('json_encode(v:_null_list)'))
+  end)
+
+  it('can dump NULL dictionary', function()
+    eq('{}', eval('json_encode(v:_null_dict)'))
+  end)
 end)