Fix JSON.create_id= to allow nil

[Watson1978]

JSON.create_id= method accepts nil as an argument in document.

oj/ext/oj/mimic_json.c

Line 655 in 2c1a622

* - *id* [_nil_|String] new create_id

However, it raises TypeError when passed nil.

irb(main):001:0> require 'oj'
=> true
irb(main):002:0> Oj.mimic_JSON
=> JSON
irb(main):003:0> JSON.create_id = nil
(irb):3:in `create_id=': wrong argument type nil (expected String) (TypeError)
        from (irb):3:in `<main>'                             
        from /home/watson/.rbenv/versions/3.2.1/lib/ruby/gems/3.2.0/gems/irb-1.6.2/exe/irb:11:in `<top (required)>'
        from /home/watson/.rbenv/versions/3.2.1/bin/irb:25:in `load'
        from /home/watson/.rbenv/versions/3.2.1/bin/irb:25:in `<main>'
irb(main):004:0> 

This patch will accept nil as an argument.

[ohler55]

arigato