fix(bindable): bindable cannot be undefined
fix(bindable): bindable cannot be undefined

Changes undefined to null to catch errors with nested compose / bindable
scenario

davismj/templating,PWKad/templating,JeroenVinke/templating,aurelia/templating,aurelia/templating,michaelgeorgeattard/templating,AshleyGrant/templating,shanonvl/aurelia-templating