Tests/RunCMake/Ninja: fix subdir prefix check

If the output path prefix is empty, the directory is going to exist.

Tests/RunCMake/Ninja/CheckNoPrefixSubDirScript.cmake

@@ -1,6 +1,6 @@
 # Check that the prefix sub-directory is not repeated.
 
-if(EXISTS "${CUR_BIN_DIR}/${NINJA_OUTPUT_PATH_PREFIX}")
+if(NINJA_OUTPUT_PATH_PREFIX AND EXISTS "${CUR_BIN_DIR}/${NINJA_OUTPUT_PATH_PREFIX}")
   message(FATAL_ERROR
     "no sub directory named after the CMAKE_NINJA_OUTPUT_PATH_PREFIX "
     "should be in the binary directory."