fix: improve error handling and nil consistency in FormatArgsJSON

- Use fmt.Sprintf fallback instead of {} on encoding errors
- Normalize nil args to {} in FormatArgsJSON for consistent output
- Update tests to expect {} instead of null for nil args

Based on PR #2670 review feedback from afjcjsbx

pkg/utils/tool_feedback_test.go

@@ -143,7 +143,7 @@ func TestFormatArgsJSON_EscapeHTMLByDefault(t *testing.T) {
 
 func TestFormatArgsJSON_NilArgs(t *testing.T) {
 	got := FormatArgsJSON(nil, false, false)
-	want := `null`
+	want := `{}`
 	if got != want {
 		t.Fatalf("FormatArgsJSON() nil = %q, want %q", got, want)
 	}