Remove nullOk from Actions.invoke, add Actions.maybeInvoke (#74680)

This removes the nullOk parameter from Actions.invoke, and adds an Actions.maybeInvoke function. The difference is that invoke will now always (in debug mode) throw if an enabled action isn't found, and maybeInvoke will return null if an enabled action isn't found. The invoke function can still return null as the result of the invocation, though, so I purposely didn't change the return value to Object from Object?.