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.

test/unit/eval/tricks_spec.lua

@@ -0,0 +1,43 @@
+local helpers = require('test.unit.helpers')
+
+local cimport = helpers.cimport
+local to_cstr = helpers.to_cstr
+local ffi = helpers.ffi
+local eq = helpers.eq
+
+local eval = cimport('./src/nvim/eval.h', './src/nvim/memory.h')
+
+local eval_expr = function(expr)
+  return ffi.gc(eval.eval_expr(to_cstr(expr), nil), function(tv)
+    eval.clear_tv(tv)
+    eval.xfree(tv)
+  end)
+end
+
+describe('NULL typval_T', function()
+  it('is produced by $XXX_UNEXISTENT_VAR_XXX', function()
+    -- Required for various tests which need to check whether typval_T with NULL
+    -- string works correctly. This test checks that unexistent environment
+    -- variable produces NULL string, not that some specific environment
+    -- variable does not exist. Last bit is left for the test writers.
+    local unexistent_env = 'XXX_UNEXISTENT_VAR_XXX'
+    while os.getenv(unexistent_env) ~= nil do
+      unexistent_env = unexistent_env .. '_XXX'
+    end
+    local rettv = eval_expr('$' .. unexistent_env)
+    eq(eval.VAR_STRING, rettv.v_type)
+    eq(nil, rettv.vval.v_string)
+  end)
+
+  it('is produced by v:_null_list', function()
+    local rettv = eval_expr('v:_null_list')
+    eq(eval.VAR_LIST, rettv.v_type)
+    eq(nil, rettv.vval.v_list)
+  end)
+
+  it('is produced by v:_null_dict', function()
+    local rettv = eval_expr('v:_null_dict')
+    eq(eval.VAR_DICT, rettv.v_type)
+    eq(nil, rettv.vval.v_dict)
+  end)
+end)