FindLAPACK: Fix support for LAPACK symbols inside BLAS libraries

Fix the condition added by commit 68dcbeee01 (FindLAPACK: Test for
implicitly linked LAPACK libraries, 2019-06-11, v3.16.0-rc1~560^2) to
use BLAS libraries if they are sufficient with no dedicated LAPACK
libraries.

Fixes: #20099

Modules/FindLAPACK.cmake

@@ -174,7 +174,7 @@ if(_libraries_work)
 endif()
 
  if(_libraries_work)
-   if("${_list}" STREQUAL "")
+   if("${_list}${_blas}" STREQUAL "")
      set(${LIBRARIES} "${LIBRARIES}-PLACEHOLDER-FOR-EMPTY-LIBRARIES")
    else()
      set(${LIBRARIES} ${${LIBRARIES}} ${_blas} ${_threads})