Fix indent when formatting empty files

[robinjmurphy]

JsFormat currently overrides the Indent Using Spaces setting when formatting empty files. This is a problem when format_on_save is enabled as it forces all new Javascript files to use tabs for indents. To reproduce the bug:

1. Create an empty file and save it with a .js extension
2. Enable the Indent Using Spaces setting in the bottom right corner of the Sublime window
3. Format the file using JsFormat (ctrl + option + f)
4. The Indent Using Spaces setting is overwritten

This is happening because:

• merge_utils.py forces translate_tabs_to_spaces to false (line 68) and later restores it to the user's chosen option (line 83)
• However, when there is no code in the file (e.g. a brand new file), the merge_code function returns early before the user's indent preferences have been restored (line 70)
• This currently results in translates_tabs_to_spaces being forced to false when saving any empty Javascript file

This PR fixes this by ensuring merge_code returns before forcing tabs if the file is empty