Fix empty prefix

[jesper-friis]

Description

Corrected handling of empty prefixes in Triplestore.expand_iri() and Triplestore.prefix_iri().
Added corresponding tests.

Type of change

• Bug fix and code cleanup
• New feature
• Documentation update
• Testing

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?
• Is the code properly tested?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.66%. Comparing base (d8ba502) to head (309b2e5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   78.40%   78.66%   +0.25%     
==========================================
  Files          18       18              
  Lines        1723     1725       +2     
==========================================
+ Hits         1351     1357       +6     
+ Misses        372      368       -4     

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[francescalb]

From reading the docstrings of prefix_iri and base_iri, it is not clear to me how these two functions are supposed to work. This needs to be explained better, perhaps adding a few examples is possible. As it is now, I do not clear when and why an error is raised.

[jesper-friis]

From reading the docstrings of prefix_iri and base_iri, it is not clear to me how these two functions are supposed to work. This needs to be explained better, perhaps adding a few examples is possible. As it is now, I do not clear when and why an error is raised.

Added examples to the docstrings of the expand_iri() and prefix_iri() methods.