Ignore empty elements in POWERLINE_CONFIG_PATHS

Ref #1201

docs/source/configuration/local.rst

@@ -144,7 +144,8 @@ above rules:
 
 Additionally one environment variable is a usual *colon*-separated list of 
 directories: ``POWERLINE_CONFIG_PATHS``. This one defines paths which will be 
-searched for configuration.
+searched for configuration. Empty paths in ``POWERLINE_CONFIG_PATHS`` are 
+ignored.
 
 .. note::
     Overrides from environment variables have lower priority then 

powerline/commands/main.py

@@ -49,7 +49,7 @@ def finish_args(environ, args):
 	if args.renderer_arg:
 		args.renderer_arg = mergeargs((parsedotval(v) for v in args.renderer_arg), remove=True)
 	args.config_path = (
-		environ.get('POWERLINE_CONFIG_PATHS', '').split(':')
+		[path for path in environ.get('POWERLINE_CONFIG_PATHS', '').split(':') if path]
 		+ (args.config_path or [])
 	)
 	return args