Analysis Algs: Support partial PRW info in input file

This is a follow-up to !59646 (merged), which updated the pileup reweighting tool to not crash if an event already contains some PRW info (such as a random run number). The pileup reweighting analysis algorithm was found to suffer from the same problem (it doesn't seem to call the function that was updated, but rather to do the same stuff manually), so this implements the same change there. The relevant accessors now check for the existence of their eventInfo decoration before attempting to write new ones.

Tagging @alory, who reported this issue.