Fix broken SPARQL syntax for PropertyConfig with qualifier and value

Qualifier support was added in 6a055c2, with support for both
Pxxx/Pzzz and Pxxx/Qyyy/Pzzz syntaxes.

The latter was however only partially covered by unit tests.
There was also no functional testing per lack of real-world use-case.

But the generated SPARQL query was incorrect:
using Pxxx/Qyyy/Pzzz will result in the SPARQL fragment
?entity p:Pxxx [ ps:Pxxx Qyyy ; pq:Pzzz [] ]
instead of the correct
?entity p:Pxxx [ ps:Pxxx wd:Qyyy ; pq:Pzzz [] ]

(That broken syntax was actually enshrined in the
unit tests added during the 5526cb378d refactoring).