fix: don't panic on invalid JSON inputs to Config

[avp]

The server is set up to deal with empty journal_file being provided in the configuration options, but not to handle configs that don't parse into a valid BeancountLspOptions.

This meant that using the default config provided in nvim-lspconfig: https://github.com/neovim/nvim-lspconfig/blob/master/lua/lspconfig/server_configurations/beancount.lua would cause a panic on an invalid unwrap() for a non-obvious reason: Lua serializes its empty table {} as the JSON array [] instead of the JSON object {}, causing the serde_json::from_value call to fail.

It seemed that beancount-language-server shouldn't panic if given a configuration that isn't useful.

Added some tests to indicate the expected behavior given these inputs.

NOTE: Ideally it should log an error or warning to the user, but I wasn't sure how exactly we should be logging it, given that returning Err from Config::update would also panic because that's currently unwrap-ed.