fix(env_tool): return None if env is empty

Signed-off-by: DragonBillow <DragonBillow@outlook.com>

src/lib.rs

@@ -2948,7 +2948,12 @@ impl Build {
     /// Returns compiler path, optional modifier name from whitelist, and arguments vec
     fn env_tool(&self, name: &str) -> Option<(PathBuf, Option<String>, Vec<String>)> {
         let tool = match self.getenv_with_target_prefixes(name) {
-            Ok(tool) => tool,
+            Ok(tool) => {
+                if tool.is_empty() {
+                    return None;
+                }
+                tool
+            },
             Err(_) => return None,
         };