Do not initialize FilePicker with empty argument.

This is tricky.
In constructor, we don't know if we were called by service manager's
createInstanceWithArgumentsAndContext or just createInstanceWithContext,
so we don't know if initialize should be called.
Let's assume that nobody would use createInstanceWithArgumentsAndContext
with empty argument. Then the behaviour should be the same as before.

FolderPicker does not seem to be called with arguments.

Change-Id: If33dc20fe1b2b10b092ff35fa0b258137c8e94b2