fix: Always overwrite with tables, even if empty

While the last commit restored behavior to mostly where it was in
v0.15.2, this commit introduces a behavior change with the goal of
treating empty tables the same as non-empty ones (in particular,
`int_to_empty` is now treated the same as `int_to_non_empty`, both of
them overwriting the integer with the table).

Treating them the same makes a lot of sense logically, and the fact that
we weren't doing so is probably a bug in the old implementation. But it
is a notable behavior change, and one worth considering carefully.

src/path/mod.rs

@@ -150,8 +150,9 @@ impl Expression {
         let parent = self.get_mut_forcibly(root);
         match value.kind {
             ValueKind::Table(ref incoming_map) => {
-                // If the parent is nil, treat it as an empty table
-                if matches!(parent.kind, ValueKind::Nil) {
+                // If the parent is not a table, overwrite it, treating it as a
+                // table
+                if !matches!(parent.kind, ValueKind::Table(_)) {
                     *parent = Map::<String, Value>::new().into();
                 }
 

tests/testsuite/merge.rs

@@ -338,8 +338,17 @@ Settings {
         .unwrap();
     let res = cfg.try_deserialize::<Settings>();
     assert_data_eq!(
-        res.unwrap_err().to_string(),
-        str!["invalid type: integer `42`, expected struct Profile for key `profile.int_to_empty`"]
+        res.unwrap().to_debug(),
+        str![[r#"
+Settings {
+    profile: {
+        "int_to_empty": Profile {
+            name: None,
+        },
+    },
+}
+
+"#]]
     );
 
     // * int_to_non_empty: int -> map with k/v