Don't drop empty object during merge

[schmidt-sebastian]

We currently drop empty objects when during set({merge: true}) since we only add leaf nodes to the field mask. We need to consider an empty object a leaf node.

Port of firebase/firebase-js-sdk#1202

[willtr101]

This caused a serious issue for us. Assuming we already have document.items, where items is a dictionary contains existing keys/values.
Now, if we update that document, with items set to an empty object {}, merge flag is true, existing items will be dropped, returning a new empty document.items, while we expect update/set with merging flag is true should keep the existing data, not wiping them {object }+ {} = {object}, not empty. Please reconsider this PR. It seems this PR cared on object.subobject where subobject is empty, keeps them as a node, however, this will fail when updating a root object.

[schmidt-sebastian]

We will discuss your comment internally. Note that you can make the behavior of set() explicit by providing the list of field paths you wish to merge. This will override any setting on our end.