fix: Treat nil parents as empty tables

Encountered this regression via nextest's test suite
(nextest-rs/nextest#2001). If a table is empty,
it would be treated as a unit value. If attempted to be deserialized
via a `Default` impl, it would lead to deserialization failing with an
error like

```console
profile.default-miri: invalid type: unit value, expected struct CustomProfileImpl
```

A bisect appears to indicate that
ec36bff is responsible. For empty
tables where the Default impl is supposed to work, it no longer would.

I've attempted to restore the old behavior of putting in an empty
table, specifically this section:

ec36bff#diff-c5423e2d2d6c87501239c0304c0f496742e00440defdd20368cf548ba42ab184L175-L178

I'm happy to make changes if there's a better approach.

I've also added some tests for a few situations around empty tables. In
this case only the last one (`profile.baz`) regressed, but I've also
added tests for a couple other cases.

src/path/mod.rs

@@ -150,12 +150,16 @@ 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) {
+                    *parent = Map::<String, Value>::new().into();
+                }
+
                 // Continue the deep merge
                 for (key, val) in incoming_map {
                     Self::root(key.clone()).set(parent, val.clone());
                 }
             }
-
             _ => {
                 *parent = value;
             }

tests/testsuite/file_toml.rs

@@ -1,5 +1,7 @@
 #![cfg(feature = "toml")]
 
+use std::collections::BTreeMap;
+
 use chrono::{DateTime, TimeZone, Utc};
 use float_cmp::ApproxEqUlps;
 use serde_derive::Deserialize;
@@ -422,6 +424,60 @@ bar = "bar is a lowercase param"
     );
 }
 
+#[test]
+fn test_override_empty_tables() {
+    #[derive(Debug, Deserialize)]
+    struct Settings {
+        profile: BTreeMap<String, Profile>,
+    }
+
+    #[derive(Debug, Default, Deserialize)]
+    struct Profile {
+        name: Option<String>,
+    }
+
+    // Test a few scenarios with empty tables:
+    // * foo: empty table -> table with k/v
+    // * bar: table with k/v -> empty table
+    // * baz: empty table relying on Default impl
+    let cfg = Config::builder()
+        .add_source(File::from_str(
+            r#"
+            [profile.foo]
+"#,
+            FileFormat::Toml,
+        ))
+        .add_source(File::from_str(
+            r#"
+            [profile.foo]
+            name = "foo"
+"#,
+            FileFormat::Toml,
+        ))
+        .add_source(File::from_str(
+            r#"
+            [profile.bar]
+            name = "bar"
+"#,
+            FileFormat::Toml,
+        ))
+        .add_source(File::from_str(
+            r#"
+            [profile.bar]
+            [profile.baz]
+"#,
+            FileFormat::Toml,
+        ))
+        .build()
+        .unwrap();
+
+    let settings: Settings = cfg.try_deserialize().unwrap();
+    assert_eq!(settings.profile.len(), 3);
+    assert_eq!(settings.profile["foo"].name.as_deref(), Some("foo"));
+    assert_eq!(settings.profile["bar"].name.as_deref(), Some("bar"));
+    assert_eq!(settings.profile["baz"].name.as_deref(), None);
+}
+
 #[test]
 fn toml() {
     let s = Config::builder()