storage-manager: get returns an error on a missing key, return None instead?

[J-Loudet]

Describe the feature

The method get:

zenoh/plugins/zenoh-backend-traits/src/lib.rs

Lines 247 to 251 in ddcc8f1

	async fn get(
	&mut self,
	key: Option<OwnedKeyExpr>,
	parameters: &str,
	) -> ZResult<Vec<StoredData>>;

returns, at least in some implementation, an error when the requested key is missing.

I believe it should return a ZResult<Option<Vec<StoredData>>> where the None indicates that there is no entry for that key expression. This would allow differentiating an actual error of the underlying Storage from a missing key.

Right now, for instance, the Memory backend returns an error: https://github.com/eclipse-zenoh/zenoh/blob/main/plugins/zenoh-plugin-storage-manager/src/memory_backend/mod.rs#L131