fix: Don't reencrypt vault with no private values using Model.build

[Geit]

We sometimes serialize a Sequelize model, and then rehydrate it using Model.build at a later time. This object is in the form:

{
// ... Some attributes ...
SubObject: [
	{
		// ... Some attributes ...
		encrypted: {}
	},
	// And many many more...
]
}

Where the encrypted field is a sequelize-encrypted vault.

As part of Model.build, Sequelize calls all custom field setters. This results in Vault.set() being called with {}, at which point it is re-encrypted in case the user ends up calling .save(). However, given the default value for a Vault call is {} anyway, this operation is pointless.

With many sub-objects, this behaviour becomes pathological and causes serious performance issues:

This PR simply skips the re-encryption operation if Vault.set() is called with an empty object (and there's no existing data to override).

Note, that this PR only helps performance in cases where the vault is empty - in cases where it is not, the existing behaviour is maintained. Ideally, (re)encryption would be deferred to the point that the user actually calls .save() on a field.

With the same data, after this PR a performance trace is much saner (44ms -> 17ms on 1500 subobjects):

[snyksec]

🎉 This PR is included in version 0.5.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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