'0' should be accepted in title field

Problem/Motivation

User should be able to use '0' in title field.
When user fills '0', the system responds unrelated error message.

Steps to reproduce

In context add/edit form,

Fill '0' in title field
Fill any concrete value in URL field

Save form and we will get error message: 'Token field is required'

Proposed resolution

Use isset() or strlen() instead of empty().

Remaining tasks

User interface changes

No.

API changes

No.

Data model changes

No.