menubutton: arrow: Use open-menu-symbolic when no direction

[ryonakano]

Backport of elementary/granite#1013 as requested at elementary/granite#1013 (comment)

Checklists

• Confirmed the expected icon is used when building our File Roller with the following diff
  
• Confirmed other widgets like dropdowns is not affected to this change
  

A patch to build this branch inside the sanbox

diff --git a/org.gnome.FileRoller.json b/org.gnome.FileRoller.json
index c3ccc8e..2c2e562 100644
--- a/org.gnome.FileRoller.json
+++ b/org.gnome.FileRoller.json
@@ -99,6 +99,65 @@
                 }
             ]
         },
+        {
+            "name": "stylesheet",
+            "buildsystem": "meson",
+            "sources": [
+                {
+                    "type": "git",
+                    "url": "https://github.com/elementary/stylesheet.git",
+                    "commit": "b87aa2020a59fdeb6aac5be0d8d27fc4ca920d01",
+                    "x-checker-data": {
+                        "type": "git",
+                        "tag-pattern": "^([\\d.]+)$"
+                    }
+                }
+            ],
+            "modules": [
+                {
+                    "name": "sassc",
+                    "cleanup": [
+                        "*"
+                    ],
+                    "sources": [
+                        {
+                            "type": "git",
+                            "url": "https://github.com/sass/sassc.git",
+                            "tag": "3.6.2"
+                        },
+                        {
+                            "type": "script",
+                            "dest-filename": "autogen.sh",
+                            "commands": [
+                                "autoreconf -si"
+                            ]
+                        }
+                    ],
+                    "modules": [
+                        {
+                            "name": "libsass",
+                            "cleanup": [
+                                "*"
+                            ],
+                            "sources": [
+                                {
+                                    "type": "git",
+                                    "url": "https://github.com/sass/libsass.git",
+                                    "tag": "3.6.5"
+                                },
+                                {
+                                    "type": "script",
+                                    "dest-filename": "autogen.sh",
+                                    "commands": [
+                                        "autoreconf -si"
+                                    ]
+                                }
+                            ]
+                        }
+                    ]
+                }
+            ]
+        },
         {
             "name": "file-roller",
             "buildsystem": "meson",

[ryonakano]

I'll separate to different file as requested at elementary/granite#1013 (review)