Fix empty vector argument array construction

[zayenz]

Summary

• Fix construction of argument arrays from empty std::vector instances by avoiding &aa[0] when the vector is empty.
• Add a regression test for empty vector construction of IntArgs, IntVarArgs, and BoolVarArgs.
• Add a changelog entry thanking @intractabilis.

Fixes #205.

Validation

• cmake -S . -B /tmp/gecode-empty-vector-args-build -DCMAKE_BUILD_TYPE=Debug -DGECODE_ENABLE_EXAMPLES=OFF -DGECODE_ENABLE_GIST=OFF -DGECODE_ENABLE_QT=OFF
• cmake --build /tmp/gecode-empty-vector-args-build --target gecode-test -j 8
• /tmp/gecode-empty-vector-args-build/bin/gecode-test -test Array::Vector::Empty
• git diff --check

[kalvdans]

Nit: aa.data() is fine to call on an empty std::vector (might or might not return nullptr). do we need the if (n > 0) then?