Always print JSON output with --format json (#3671)

## Summary

Closes https://github.com/astral-sh/uv/issues/3670.

## Test Plan

```
uv on  charlie/list:main
❯ cargo run pip list --editable
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv pip list --editable`
uv on  charlie/list:main
❯ cargo run pip list --editable --format json
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running `target/debug/uv pip list --editable --format json`
[]
uv on  charlie/list:main
❯ cargo run pip list --editable --format freeze
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv pip list --editable --format freeze`
uv on  charlie/list:main
❯ cargo run pip list --editable --format columns
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv pip list --editable --format columns`
```

crates/uv/src/commands/pip/list.rs

@@ -65,11 +65,14 @@ pub(crate) fn pip_list(
         .filter(|dist| !exclude.contains(dist.name()))
         .sorted_unstable_by(|a, b| a.name().cmp(b.name()).then(a.version().cmp(b.version())))
         .collect_vec();
-    if results.is_empty() {
-        return Ok(ExitStatus::Success);
-    }
 
     match format {
+        ListFormat::Json => {
+            let rows = results.iter().copied().map(Entry::from).collect_vec();
+            let output = serde_json::to_string(&rows)?;
+            writeln!(printer.stdout(), "{output}")?;
+        }
+        ListFormat::Columns if results.is_empty() => {}
         ListFormat::Columns => {
             // The package name and version are always present.
             let mut columns = vec![
@@ -111,11 +114,7 @@ pub(crate) fn pip_list(
                 writeln!(printer.stdout(), "{}", elems.join(" ").trim_end())?;
             }
         }
-        ListFormat::Json => {
-            let rows = results.iter().copied().map(Entry::from).collect_vec();
-            let output = serde_json::to_string(&rows)?;
-            writeln!(printer.stdout(), "{output}")?;
-        }
+        ListFormat::Freeze if results.is_empty() => {}
         ListFormat::Freeze => {
             for dist in &results {
                 writeln!(

crates/uv/tests/pip_list.rs

@@ -36,12 +36,14 @@ fn install_command(context: &TestContext) -> Command {
 }
 
 #[test]
-fn list_empty() {
+fn list_empty_columns() {
     let context = TestContext::new("3.12");
 
     uv_snapshot!(Command::new(get_bin())
         .arg("pip")
         .arg("list")
+        .arg("--format")
+        .arg("columns")
         .arg("--cache-dir")
         .arg(context.cache_dir.path())
         .env("VIRTUAL_ENV", context.venv.as_os_str())
@@ -56,6 +58,53 @@ fn list_empty() {
     );
 }
 
+#[test]
+fn list_empty_freeze() {
+    let context = TestContext::new("3.12");
+
+    uv_snapshot!(Command::new(get_bin())
+        .arg("pip")
+        .arg("list")
+        .arg("--format")
+        .arg("freeze")
+        .arg("--cache-dir")
+        .arg(context.cache_dir.path())
+        .env("VIRTUAL_ENV", context.venv.as_os_str())
+        .env("UV_NO_WRAP", "1")
+        .current_dir(&context.temp_dir), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    "###
+    );
+}
+
+#[test]
+fn list_empty_json() {
+    let context = TestContext::new("3.12");
+
+    uv_snapshot!(Command::new(get_bin())
+        .arg("pip")
+        .arg("list")
+        .arg("--format")
+        .arg("json")
+        .arg("--cache-dir")
+        .arg(context.cache_dir.path())
+        .env("VIRTUAL_ENV", context.venv.as_os_str())
+        .env("UV_NO_WRAP", "1")
+        .current_dir(&context.temp_dir), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    []
+
+    ----- stderr -----
+    "###
+    );
+}
+
 #[test]
 fn list_single_no_editable() -> Result<()> {
     let context = TestContext::new("3.12");
@@ -451,6 +500,7 @@ fn list_format_json() {
     success: true
     exit_code: 0
     ----- stdout -----
+    []
 
     ----- stderr -----
     "###