topdown: move arg count below ir.Empty()

...to avoid a panic at run time.

Fixes open-policy-agent#3501.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

test/cases/testdata/functions/test-functions-unused-arg.yaml

@@ -0,0 +1,12 @@
+cases:
+- data:
+  modules:
+  - |
+    package p
+    
+    f(x) {
+        r = input.that_is_not_there
+    }
+  note: basic call
+  query: data.p.f(1)
+  want_result: []

topdown/eval.go

@@ -1506,12 +1506,12 @@ func (e evalFunc) eval(iter unifyIterator) error {
 		return err
 	}
 
-	argCount := len(ir.Rules[0].Head.Args)
-
 	if ir.Empty() {
 		return nil
 	}
 
+	argCount := len(ir.Rules[0].Head.Args)
+
 	if len(ir.Else) > 0 && e.e.unknown(e.e.query[e.e.index], e.e.bindings) {
 		// Partial evaluation of ordered rules is not supported currently. Save the
 		// expression and continue. This could be revisited in the future.