* hash.c: separate NULL and EMPTY check functions.
  `RHASH_TABLE_EMPTY` function checks NULL table or not,
  but it should be named "NULL_P".
  Introduce `RHASH_TABLE_EMPTY_P` function to check size == 0.
  There are cases that hash has table data even if data is not NULL
  (in case removed after inserted elements).


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66392 b2dd03c8-39d4-4d8f-98ff-823fe69b080e