fix(combos): Properly report combos len with emply block (#2739)

Handle the scenario where there is an empty combos block and return
a zero combos length.

app/tests/pointing/mouse-move/processors/behaviors_hold_tap/native_posix_64.keymap

@@ -18,6 +18,10 @@
 };
 
 / {
+    combos {
+        compatible = "zmk,combos";
+    };
+
     keymap {
         compatible = "zmk,keymap";
         label ="Default keymap";