Only apply filter if filter is set

[nickgzzjr]

This is an attempt to fix Issue 1348

[niklassaers]

Actually, need to test for an empty filter name too. Having set no filter, I get "" here, not nil

[cobarx]

I've refactored the code so that we handle both empty string and nil values. I also made it keep track of whether we've initialized the filter or not so that you can turn off the filter after you've applied a filter. I believe this was lost in the new PR.

[niklassaers]

👍 Thank you. 🙏

[nickgzzjr]

@niklassaers I had specifically made it to test filter != nil only and not "" because on some videos (with rotated orientations) the transformation matrix does not get applied correctly to the videoComposition if doesn't start playing with a filter. I think what @cobarx had suggested in Issue 1348 to add filterEnabled is the way to go. I'll try to get a pull request in later this evening.