grn_raw_string: use "cstring == NULL" to avoid "convert char[] to bool" warning
grn_raw_string: use "cstring == NULL" to avoid "convert char[] to bool" warning

kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga