db/SessionStore.js: call nextTick() only if there is something to do
db/SessionStore.js: call nextTick() only if there is something to do

Changed two occurrences of:

  process.nextTick(function() {
     if (fn) fn();
  });

with

  if (fn) {
    process.nextTick(fn);
  }

i.e. such that no function even gets added to the `nextTick` queue unless
there's actually a function to be called.

Extracted from Ray's work.

aptivate/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite