Fix Ruby wrapper build errors

[pfusik]

ruby/ohcount_wrap.c:4640:9: error: incompatible pointer to integer conversion initializing 'VALUE'
      (aka 'unsigned long') with an expression of type 'void *' [-Wint-conversion]
 4640 |   VALUE arg2 = NULL ;
      |         ^      ~~~~
 ruby/ohcount_wrap.c:4845:9: error: incompatible pointer to integer conversion initializing 'VALUE'
      (aka 'unsigned long') with an expression of type 'void *' [-Wint-conversion]
 4845 |   VALUE arg1 = NULL ;
      |         ^      ~~~~

https://docs.ruby-lang.org/capi/en/master/d4/d3f/value_8h.html#af5fa1600fbcceb5c1bbe27dd3eaab113

says:

typedef uintptr_t VALUE

so zero is more appropriate than NULL.