Empty Tree test case

[andrijamitrovic23]

Overview

This is a PR for addition of new test cases that have been noticed to be missing through the audit of the nmt repository. The intention is to increase the number of test cases by adding tests for edge cases and for the issues that have been reported.

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov]

Codecov Report

Merging #149 (5f30795) into master (eb012d0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   96.20%   96.20%           
=======================================
  Files           6        6           
  Lines         527      527           
=======================================
  Hits          507      507           
  Misses         12       12           
  Partials        8        8           

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