test/environ_spec: Windows treats empty as undefined

ref #10657

test/functional/eval/environ_spec.lua

@@ -3,6 +3,7 @@ local clear = helpers.clear
 local eq = helpers.eq
 local environ = helpers.funcs.environ
 local exists = helpers.funcs.exists
+local iswin = helpers.iswin
 
 describe('environment variables', function()
   it('environ() handles empty env variable', function()
@@ -11,8 +12,8 @@ describe('environment variables', function()
     eq(nil, environ()['DOES_NOT_EXIST'])
   end)
   it('exists() handles empty env variable', function()
-    clear({env={EMPTY_VAR=""}})
-    eq(1, exists('$EMPTY_VAR'))
+    clear({env={EMPTY_VAR=""}})  -- Windows treats this as "undefined".
+    eq((iswin() and 0 or 1), exists('$EMPTY_VAR'))
     eq(0, exists('$DOES_NOT_EXIST'))
   end)
 end)