Skip empty GC information

[okumin]

In our environment, LastGcInfo is typically empty as the list retains entries where no GC has happened. The current implementation of gc method returns nil in that case.

This is a demo.

irb(main):017:0> def mofu(arr)
irb(main):018:1>   result = arr.map { |x|
irb(main):019:2*     return nil if x.nil?
irb(main):020:2>     x + 1
irb(main):021:2>   }
irb(main):022:1> end
=> :mofu
irb(main):023:0> mofu([1, 2, 3])
=> [2, 3, 4]
irb(main):024:0> mofu([1, nil, 3])
=> nil

The new information should return the elements of valid entries.

irb(main):043:0> def mofu(arr)
irb(main):044:1>   arr.reject { |x| x.nil? }.map { |x|
irb(main):045:2*     x + 1
irb(main):046:2>   }
irb(main):047:1> end
=> :mofu
irb(main):048:0> mofu([1, 2, 3])
=> [2, 3, 4]
irb(main):049:0> mofu([1, nil, 3])
=> [2, 4]