FindArmadillo: Remove unnecessary setting of ARMADILLO_LIBRARY to empty This is not a typical pattern for find modules, and it breaks subsequent calls to `find_package(Armadillo)`. See also commit 38c6dd7b19 (FindArmadillo: Add support for non-wrapper case, 2020-03-02, v3.18.0-rc1~594^2) and commit 6745af99c8 (FindArmadillo: Fix wrapper linking with MSVC, 2025-02-05, v4.0.0-rc1~53^2).