UseJava: fix find_jar() called with multiple files (#13281)

Modules/UseJava.cmake

@@ -533,9 +533,9 @@ function (find_jar VARIABLE)
         endif (${_state} STREQUAL "name")
     endforeach (arg ${ARGN})
 
-    if (${_jar_names} STREQUAL "")
+    if (NOT _jar_names)
         message(FATAL_ERROR "find_jar: No name to search for given")
-    endif (${_jar_names} STREQUAL "")
+    endif (NOT _jar_names)
 
     foreach (jar_name ${_jar_names})
         foreach (version ${_jar_versions})