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-command/src/formats/to/json.rs

@@ -142,7 +142,10 @@ pub fn value_to_json_value(v: &Value) -> Result<nu_json::Value, ShellError> {
             let collected = val.collect()?;
             value_to_json_value(&collected)?
         }
-        Value::CustomValue { val, .. } => val.to_json(),
+        Value::CustomValue { val, span } => {
+            let collected = val.to_base_value(*span)?;
+            value_to_json_value(&collected)?
+        }
     })
 }