fix weird crash where some MS2s have no precursors (#1618)
fix weird crash where some MS2s have no precursors (#1618)

* fix weird crash where some MS2s have no precursors

* whoops

lschaffer2/MetaMorpheus,zrolfs/MetaMorpheus,smith-chem-wisc/MetaMorpheus,lonelu/MetaMorpheus,rmillikin/MetaMorpheus