Fix handling of empty segment includes

[jovyntls]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Fix an edge case in handling includes for empty segments - the desired result is to allow the empty segment to be included instead of throwing an error.

Testing instructions:
Minimal reproduction:

<div id="empty"></div>
<include src="#empty">

Proposed commit message: (wrap lines at 72 characters)

Fix handling of empty segment includes

Attempting to include an empty segment will throw a "hash not found"
error on the MarkBind console output.

This is a regression on existing behaviour introduced by refactoring on
includePanelProcessor.

Let's revert the original behaviour of allowing empty hashes to be
included, and add test cases to check for this.

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[tlylt]

LGTM

[tlylt]

Let's put up an issue for discussion on the suggestion that if the fragment is empty, perhaps we can consider outputting a warning since that could be a case of incorrect usage.