The if statement is good because that one ISR handles interrupts from 6 different sources. Even if you enable just the one, I like having some kind of code to indicate that to the developer. An if statement, or even an assert statement that none of the other relevant bits are set, or whatever works for the application.
And writing a 1 to PR1 clears the interrupt (clears the bit). Writing a 0 has no effect.
Did you have a chance to try enabling the interrupt from a task? In that test, you should see FreeRTOS start successfully I think.
Jeff