fix: OpenIDConnectFrontend: check for empty db_uri

[vladimir-mencl-eresearch]

With IdentityPython/pyop#44 merged, OpenIDConnectFrontend init fails when db_uri is not set, as StorageBase.type now throws a ValueError for db_uri values that do not match one of the recognised storage types (including when db_uri is None).

Fix this by guarding the StorageBase.type with a pythonic test whether db_uri was provided.

Same test already guards StorageBase.from_uri, add it also to the StorageBase.type call made to determine self.stateless.

PS: This worked before IdentityPython/pyop#44 was merged because StorageBase.type used to return the ValueError object, which evaluated as not matching string "stateless". I also thought about changing StorageBase.type to return None for db_uri of value None, but I think it's cleaner to handle it this way and keep the StorageBase.type contract clear: Only valid values allowed.

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[c00kiemon5ter]

Sounds good - thank you!