find_dependency: Give more helpful message if VERSION is empty

Modules/CMakeFindDependencyMacro.cmake

@@ -31,14 +31,17 @@ macro(find_dependency dep)
   if (NOT ${dep}_FOUND)
     set(cmake_fd_version)
     if (${ARGC} GREATER 1)
-      if (${ARGV1} STREQUAL EXACT)
+      if ("${ARGV1}" STREQUAL "")
+        message(FATAL_ERROR "Invalid arguments to find_dependency. VERSION is empty")
+      endif()
+      if ("${ARGV1}" STREQUAL EXACT)
         message(FATAL_ERROR "Invalid arguments to find_dependency. EXACT may only be specified if a VERSION is specified")
       endif()
       set(cmake_fd_version ${ARGV1})
     endif()
     set(cmake_fd_exact_arg)
     if(${ARGC} GREATER 2)
-      if (NOT ${ARGV2} STREQUAL EXACT)
+      if (NOT "${ARGV2}" STREQUAL EXACT)
         message(FATAL_ERROR "Invalid arguments to find_dependency")
       endif()
       set(cmake_fd_exact_arg EXACT)