Block Context assignment form element rendered when only null option is available

Problem/Motivation

The ContextAwarePluginAssignmentTrait::addContextAssignmentElement(), when it collects $options, can end up with empty options in certain circumstances. This ends up displaying an empty select list on the block form.

Steps to reproduce

Two examples where [view_mode: null] is only option:
1. Create views block with a contextual filter on a plain text field. Add the views block via layout builder and form shows an empty select list.
2. Create views block with a contextual filter on Taxonomy term: Name via a relationship to a term. Add the views block via layout builder and form shows an empty select list.

Proposed resolution

Filter out null $options in addContextAssignmentElement()

Remaining tasks

Create patch to adjust ContextAwarePluginAssignmentTrait::addContextAssignmentElement()

User interface changes

Does not create select list for contextual filters on block form on null options.

API changes

none

Data model changes

none