Allow upsert to update nodes instead of overwriting them

[destructivedata]

Current Situation

In its current version the method upsert_node() simply overwrites a node if it already exists.

Possible Improvement

Nodes should only be overwritten if explicitly specified, the new default could e.g. add the new values if the field of the existing node is null. An even more elaborate approach could allow for a white-/blacklist of keys that can be overwritten.

Example

upsert_node(
    self,
    collection_name: str,
    data: dict[str, Any],
    overwrite: bool = False,
    alt_key: str | list[str] | None = None
) -> Document:
    # ...
    if node := self.get_document(collection_name, data=data, alt_key=alt_key):
        # Update existing vertex
        for key, value in data.items():
            if node[key] is None and value is not None:
                node[key] = value

        # Save updated node
        node.patch()
    # ...

Additional Considerations

The datasource key is currently a single str, if multiple datasources contribute to the contents of a node consider list[str] instead.