Fix published frame content for sequence starting with 0

[kalisp]

Brief description

Previously expression didn't trigger as repre.get("frameStart") returned 0 which translated into False

Description

This resulted in using frame value in source padding which might had been different from final, correct value.

Additional info

In AE it manifested when artist manually updated frame pattern in Render Queue, used more # than is in final pattern from Anatomy (which is ####). AE rendered locally files with 00000 temporarily, integrate_new used correct padding for publishing, but representation.context.frame stayed with wrong value (00000 instead of 0000).
This might have happened in another hosts too.

Testing notes:

1. add new Render Queue in AE, choose "PNG" as destination format
2. modify name there to contain more or less # than 4
3. render locally
4. check published files, they will be correct
5. check representation in DB (db.getCollection('petr_test').find({type:"representation", name:"png"}).sort({"_id": -1})) , its context.frame will match source pattern
6. Try to load this published representation (you can load it in AE directly) - it should fail

[mkolar]

Task linked: OP-2049 after effects publishing- frame padding fix