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/dataframe/values/nu_dataframe/custom_value.rs

@@ -33,10 +33,6 @@ impl CustomValue for NuDataFrame {
         Ok(Value::List { vals, span })
     }
 
-    fn to_json(&self) -> nu_json::Value {
-        nu_json::Value::Null
-    }
-
     fn as_any(&self) -> &dyn std::any::Any {
         self
     }

crates/nu-command/src/dataframe/values/nu_expression/custom_value.rs

@@ -34,10 +34,6 @@ impl CustomValue for NuExpression {
         Ok(self.to_value(span))
     }
 
-    fn to_json(&self) -> nu_json::Value {
-        nu_json::Value::Null
-    }
-
     fn as_any(&self) -> &dyn std::any::Any {
         self
     }

crates/nu-command/src/dataframe/values/nu_lazyframe/custom_value.rs

@@ -47,10 +47,6 @@ impl CustomValue for NuLazyFrame {
         Ok(Value::Record { cols, vals, span })
     }
 
-    fn to_json(&self) -> nu_json::Value {
-        nu_json::Value::Null
-    }
-
     fn as_any(&self) -> &dyn std::any::Any {
         self
     }

crates/nu-command/src/dataframe/values/nu_lazygroupby/custom_value.rs

@@ -38,10 +38,6 @@ impl CustomValue for NuLazyGroupBy {
         Ok(Value::Record { cols, vals, span })
     }
 
-    fn to_json(&self) -> nu_json::Value {
-        nu_json::Value::Null
-    }
-
     fn as_any(&self) -> &dyn std::any::Any {
         self
     }

crates/nu-command/src/dataframe/values/nu_when/custom_value.rs

@@ -34,10 +34,6 @@ impl CustomValue for NuWhen {
         Ok(value)
     }
 
-    fn to_json(&self) -> nu_json::Value {
-        nu_json::Value::Null
-    }
-
     fn as_any(&self) -> &dyn std::any::Any {
         self
     }