Fix resolving backend.context in cypher

[mszostok]

Description

Changes proposed in this pull request:

• Fix resolving backend.context in cypher. Currently, the context was always taken from the user input even if the backend was not sent. This against our contract, in case of empty backend we should use the old backend.context

Update scenario:

• if backend is not sent, use old settings
• if backend was sent, use new backend.context value (if it's null then persist null which means, zero backend.context property)

Testing

1. Run neo4j:

   docker run -d \
     -p 7687:7687 -p 7474:7474 \
     -e "NEO4J_AUTH=neo4j/okon" \
     -e "NEO4JLABS_PLUGINS=[\"apoc\"]" \
     --name hub-neo4j-instance \
     ghcr.io/capactio/neo4j:4.2.13-apoc

2. Run storage server:

   APP_LOGGER_LEVEL="debug" APP_LOGGER_DEV_MODE=true APP_SUPPORTED_PROVIDERS="dotenv" go run ./cmd/secret-storage-backend/main.go

3. Run Local Hub:

   cd hub-js; APP_NEO4J_ENDPOINT=bolt://localhost:7687 APP_NEO4J_PASSWORD=okon APP_HUB_MODE=local npm run dev; cd ..

4. Run test:

   GRPC_SECRET_STORAGE_BACKEND_ADDR="0.0.0.0:50051" go test ./pkg/hub/client/local/ -v -count 1

You can also manually test that using GraphQL playground.

[pkosiec]

LGTM with one improvement suggestion. If that's not something easy to do, let's skip it and merge this PR 👍