Repro steps:

1. Open 05_Non-RandomizedEmission scene from the [Particles] FTP project.
2. Observe the Bursts (infinite) and Bursts (finite) effects.
3. Select them and observe that only the first of their bursts is set to 1.

Expected outcome:
All probability fields to upgrade to "1".

Notes:
- regression introduced when the Burst Probability improvement landed in 18.3 trunk
- the original prefab burst probability upgraded to "1", but additional bursts were added to the instance, and those did not upgrade to "1".

RichardK Note:
- we need to add some code to PrefabMerging.cpp to handle this new property