Fix a small bug in the TurtleParser

[joka921]

Since #1301, triples were the subject is an explicit blank node (for example _:x <is-a> <blankNode>.) could no longer be parsed. This commit fixes this bug and adds a unit test for this case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.10%. Comparing base (7ad3f58) to head (31bc56e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1311   +/-   ##
=======================================
  Coverage   88.09%   88.10%           
=======================================
  Files         313      313           
  Lines       28503    28503           
  Branches     3155     3155           
=======================================
+ Hits        25110    25112    +2     
  Misses       2235     2235           
+ Partials     1158     1156    -2     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[hannahbast]

Thanks a lot for this quick fix! It looks good + I built indexes with it for several datasets (where it didn't work without the fix) and everything worked fine.