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).

Modules/FindArmadillo.cmake

diff --git a/Modules/FindArmadillo.cmake b/Modules/FindArmadillo.cmake
index 901ea22..861540b 100644
--- a/Modules/FindArmadillo.cmake
+++ b/Modules/FindArmadillo.cmake
@@ -133,7 +133,7 @@
   set(Armadillo_VERSION_NAME "EARLY RELEASE")
 
   if(EXISTS "${ARMADILLO_INCLUDE_DIR}/armadillo_bits/arma_version.hpp")
-    # Read and parse armdillo version header file for version number
+    # Read and parse Armadillo version header file for version number.
     file(STRINGS "${ARMADILLO_INCLUDE_DIR}/armadillo_bits/arma_version.hpp" _ARMA_HEADER_CONTENTS REGEX "#define ARMA_VERSION_[A-Z]+ ")
     string(REGEX REPLACE ".*#define ARMA_VERSION_MAJOR ([0-9]+).*" "\\1" ARMADILLO_VERSION_MAJOR "${_ARMA_HEADER_CONTENTS}")
     string(REGEX REPLACE ".*#define ARMA_VERSION_MINOR ([0-9]+).*" "\\1" ARMADILLO_VERSION_MINOR "${_ARMA_HEADER_CONTENTS}")
@@ -176,8 +176,6 @@
     )
   mark_as_advanced(ARMADILLO_LIBRARY)
   set(_ARMA_REQUIRED_VARS ARMADILLO_LIBRARY)
-else()
-  set(ARMADILLO_LIBRARY "")
 endif()
 
 # Transitive linking with the wrapper does not work with MSVC,