cache: provide at least one value in parser

Fix #1747

dvc/command/cache.py

@@ -53,7 +53,6 @@ def add_parser(subparsers, parent_parser):
     )
     cache_dir_parser.add_argument(
         "value",
-        nargs="?",
         default=None,
         help="Path to cache directory. Relative paths are resolved relative "
         "to the current directory and saved to config relative to the "

tests/test_cache.py

@@ -128,6 +128,10 @@ class TestCacheLinkType(TestDvc):
 
 
 class TestCmdCacheDir(TestDvc):
+    def test(self):
+        ret = main(["cache", "dir"])
+        self.assertEqual(ret, 254)
+
     def test_abs_path(self):
         dname = os.path.join(os.path.dirname(self._root_dir), "dir")
         ret = main(["cache", "dir", dname])