cli: respect $NO_COLOR only if it's non-empty

This is what the spec says.

CHANGELOG.md

@@ -18,6 +18,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed bugs
 
+* The `$NO_COLOR` environment variable must now be non-empty to be respected.
+
 ## [0.24.0] - 2024-12-04
 
 ### Release highlights

cli/src/config.rs

@@ -359,7 +359,7 @@ pub fn config_from_environment(default: config::Config) -> StackedConfig {
 /// Environment variables that should be overridden by config values
 fn env_base() -> config::Config {
     let mut builder = config::Config::builder();
-    if env::var("NO_COLOR").is_ok() {
+    if !env::var("NO_COLOR").unwrap_or_default().is_empty() {
         // "User-level configuration files and per-instance command-line arguments
         // should override $NO_COLOR." https://no-color.org/
         builder = builder.set_override("ui.color", "never").unwrap();