Documentation, check if config dir exists

doc/userscripts.asciidoc

@@ -33,6 +33,7 @@ The following environment variables will be set when a userscript is launched:
 - `QUTE_FIFO`: The FIFO or file to write commands to.
 - `QUTE_HTML`: Path of a file containing the HTML source of the current page.
 - `QUTE_TEXT`: Path of a file containing the plaintext of the current page.
+- `QUTE_CONFIG_DIR`: Path of the directory containing qutebrowser's configuration.
 
 In `command` mode:
 

qutebrowser/commands/userscripts.py

@@ -376,7 +376,9 @@ def run(cmd, *args, win_id, env, verbose=False):
     user_agent = config.get('network', 'user-agent')
     if user_agent is not None:
         env['QUTE_USER_AGENT'] = user_agent
-    env['QUTE_CONFIG_DIR'] = standarddir.config()
+    config_dir = standarddir.config()
+    if config_dir is not None:
+        env['QUTE_CONFIG_DIR'] = config_dir
     cmd_path = os.path.expanduser(cmd)
 
     # if cmd is not given as an absolute path, look it up