New subtitle file translation is prepopulated with source language

**Describe the bug**

Creating a new translation of a .ass subtitle file copies all source strings instead of starting blank.

This is possibly because 'Monolingual base language file' is required to be set for subtitle files?

**To Reproduce the bug**

1. Set up a .ass subtitle translation component
2. Create a new translation
3. Note that the new translation's strings are filled with duplicates of the original (fortunately are flagged as warning)

**Screenshots**



**Additional Info**

Also, just a note this does not appear to be a sub2po issue, as when sub2po is run standalone, it does not prepopulate msgstr:

```
#. visible for 3 seconds
#: 00:00:23.250--%3E00:00:26.940
msgid "If you pull this off, they're willing to let you come back."
msgstr ""
```