Bug fix: hasMany loaded as null is throwing exception

If a hasMany relationship is loaded as `null`, don't throw `Adapter is either null or does not implement 'findHasMany' method`. I have written some failing tests for this situation. I also committed a proposed fix, but not sure if it is the right approach.



Change introduced at this commit https://github.com/emberjs/data/commit/81e6fdeef02e909c90b88da93d94c5cab7625baf for these files:
`system/relationships/has_many.js:20`
`system/store.js:679`