Fix \Drupal\taxonomy\Entity\Term::getName() to conform to the interface

Problem/Motivation

Deprecated function: trim(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\taxonomy\TermForm->buildEntity()

Steps to reproduce

1. Add a field that uses ajax in its widget to a vocabulary (for example file/image or a field with unlimited cardinality)
2. Go to the add term form
3. Trigger an ajax operation without entering a term name

Proposed resolution

Change \Drupal\taxonomy\Entity\Term::getName to conform to the interface and always return a string.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet