Fix exception on parsing empty document with syck

On parsing an empty YAML document on ree-1.8.7 (syck backend) was getting an exception:

NoMethodError: undefined method `value' for false:FalseClass

Triggered by the method SafeYAML::Resolver#resolve_node being called with "false" as its parameter. This happens where no root node is present.

Have added a check that if this method is called with false or nil it will return its argument unaltered. Result for an empty document seems to be consistent on psych - false in both cases.