Investors worry that Realty Income has some troubled tenants, but it's not as big a deal as it may appear.An increasing number of retailers have fallen on hard times in some way. The list includes Rite Aid, Red Lobster, Walgreens (WBA 0.34%), Dollar Tree (DLTR -2.88%), At Home, and Big Lots (BIG 9.12%). For landlords, the problems here -- from outright bankruptcies to plans to close underperforming stores -- mean lost rent.Realty Income (O 0.11%) is no different from any other landlord in this regard. But e ...