fix: handle null values in Vault secrets to prevent NullReference

[MarcelaCarvalho]

This PR fixes an issue where the configuration provider throws a NullReferenceException when Vault secrets contain keys with null values. The root cause was an invalid cast of a null value to JsonElement.

The fix adds a null check and creates a JsonElement representing a JSON null (JsonValueKind.Null) when the value is null. This ensures that:

• Keys with null values are properly included in the configuration.

• No exceptions are thrown during configuration loading.

• Consumers of IConfiguration can safely detect and handle keys with null values.

Issue: NullReferenceException when Vault secret contains null values in keys #70