Bug fix: _toAbsolutePath choked on null relative path

Author: nathancarter

.gitignore

@@ -1,7 +1,8 @@
+*.DS_Store
 node_modules/
 jsfs.map
 *.swp
 tests/*-spec.js
 .atom-build.json
 demo/*.js
-demo/*.map
+demo/*.map

jsfs.litcoffee

@@ -187,6 +187,7 @@ If this function is given an absolute path as its second argument,
 it returns it unchanged.
 
         _toAbsolutePath : ( cwdPath, relativePath ) ->
+            if not relativePath? then return cwdPath
             sep = FileSystem::pathSeparator
             if relativePath[...sep.length] is sep then return relativePath
             result = FileSystem::_joinPath \