Primarily, this gives a way to explicitly disable eviction, which is
necessary to use omitempty on EvictionHard.
See: https://github.com/kubernetes/kubernetes/pull/53833#discussion_r166672137

As justification for this approach, neither 0% nor 100% make sense as
eviction thresholds; in the "less-than" case, you can't have less than
0% of a resource and 100% perpetually evicts; in the
"greater-than" case (assuming we ever add a resource with this
semantic), the reasoning is the reverse (not more than 100%, 0%
perpetually evicts).