Fix to json for SQLite databases (#8343)

Fixes #8341. 

The `CustomValue::to_json()` function is an odd duck; it defaults to
returning `null`, and no `CustomValue` implementations override it to do
anything useful. I forgot to implement `to_json()` for `SQLiteDatabase`,
so `open foo.db | to json` was returning `null`.

To fix this, I've removed `CustomValue::to_json()` and now `to json`
will collect a `CustomValue` into a regular `Value` before doing a JSON
conversion.

crates/nu-protocol/src/value/custom_value.rs

@@ -17,11 +17,6 @@ pub trait CustomValue: fmt::Debug + Send + Sync {
     // That already exist in nushell
     fn to_base_value(&self, span: Span) -> Result<Value, ShellError>;
 
-    // Json representation of custom value
-    fn to_json(&self) -> nu_json::Value {
-        nu_json::Value::Null
-    }
-
     // Any representation used to downcast object to its original type
     fn as_any(&self) -> &dyn std::any::Any;