Handling of null or undefined param in object.merge

Due to the while conditional in object.merge, there is a nasty edge-case which is about null or undefined in object.merge:

merge({}, null, {foo: 'bar'}) ==> {}
merge({}, {foo: 'bar'}, null) ==> {foo: 'bar'}

Which is counter-intuitive on how it is on Object.assign:

Object.assign({}, null, {foo: 'bar'}) ==> {foo: 'bar'}
Object.assign({}, {foo: 'bar'}, null) ==> {foo: 'bar'}

The same problems exist for undefined instead of null.

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