Publish without arguments

[IlyaSemenov]

Currently the second argument in publish(channel, arg) is required. This contradict the typical event emitter pattern where the argument is not necessary required (sometimes only the event itself matters). I propose to make the second argument optional. If it's required due to downstream implementation details, I propose to make it equal to null by default.

See demos:

1) works fine when sending null:

> PGPubsub = require('pg-pubsub'); new PGPubsub('postgres://localhost/xxx').publish('foo', null)
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events:
      { removeListener: [Function: updateExceptionCapture],
        newListener: [Function: updateExceptionCapture],
        error: [Function: debugDomainError] },
     _eventsCount: 3,
     _maxListeners: undefined,
     members: [] } }
> Successful retry attempt for pubsub

Throws when sending without arguments:

> PGPubsub = require('pg-pubsub'); new PGPubsub('postgres://localhost/xxx').publish('foo')
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events:
      { removeListener: [Function: updateExceptionCapture],
        newListener: [Function: updateExceptionCapture],
        error: [Function: debugDomainError] },
     _eventsCount: 3,
     _maxListeners: undefined,
     members: [] } }
> Successful retry attempt for pubsub
(node:56886) UnhandledPromiseRejectionWarning: VError: Failed to publish to channel: syntax error at or near "NULL"
    at _getDB.then.catch.err (/Users/semenov/xxx/node_modules/pg-pubsub/index.js:126:34)
    at process._tickCallback (internal/process/next_tick.js:68:7)
(node:56886) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 4)