compiler: write_sync_server_create: Fix no methds warn

If sync server doesn't have any methods, the generated code will get
warn.
This commit adds check to not add "mnt" to handle this issue.

Fixes: #273

Signed-off-by: Hui Zhu <teawater@antgroup.com>

Author: teawater

compiler/src/codegen.rs

@@ -574,8 +574,13 @@ impl<'a> ServiceGen<'a> {
             method_handler_name,
         );
 
+        let has_normal_method = self.has_normal_method();
         w.pub_fn(&s, |w| {
-            w.write_line("let mut methods = HashMap::new();");
+            if has_normal_method {
+                w.write_line("let mut methods = HashMap::new();");
+            } else {
+                w.write_line("let methods = HashMap::new();");
+            }
             for method in &self.methods[0..self.methods.len()] {
                 w.write_line("");
                 method.write_bind(w);