Fix condition that use potential undefined variable and crash the application

[Seao]

Bug

It's possible to define pieces selected through the property by in custom pieces widget.
However set by to ['all'] crashs the application, which try to read a property of an undefined value in an if condition.

How to reproduce

Create a custom pieces widget extending apostrophe-pieces-widgets with the following by restriction:

module.exports = {
  extend: 'apostrophe-pieces-widgets',
  by: ['all'],
  ...
}

Try to run the application, which will crash with the following error

TypeError: Cannot read property 'relationship' of undefined
    at Object.self.pushPieceForWidget (/Users/seao/Travail/Stage/Michelin/bicycle/node_modules/apostrophe/lib/modules/apostrophe-pieces-widgets/index.js:297:24)
    at ...

Fix

Change

if (self.joinById.relationship && (widget.by === 'id'))

to

if (self.joinById && self.joinById.relationship && (widget.by === 'id'))

as self.joinById can be undefined

[boutell]

Thanks!