topdown: deal with default functions (#3505)

They aren't supported, but they can lead to a situation where
ir.Empty() is false, but ir.Rules still has length 0.

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

internal/wasm/sdk/test/e2e/exceptions.yaml

@@ -1 +1,2 @@
 # Exception Format is <test name>: <reason>
+"functions/default": "not supported in topdown, https://github.com/open-policy-agent/opa/issues/2445"

test/cases/testdata/functions/test-functions-default.yaml

@@ -0,0 +1,21 @@
+cases:
+- data:
+  modules:
+  - |
+    package p.m
+
+    default hello = false
+
+    hello() = m {
+      m = input.message
+      1 == 2
+      m = "world"
+    }
+    h = m {
+      m = hello()
+    }
+  note: functions/default # not supported but shouldn't panic
+  query: data.p.m = x
+  want_result:
+  - x:
+      hello: false

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

@@ -7,6 +7,6 @@ cases:
     f(x) {
         r = input.that_is_not_there
     }
-  note: basic call
+  note: unused arg
   query: data.p.f(1)
   want_result: []

topdown/eval.go

@@ -1506,7 +1506,9 @@ func (e evalFunc) eval(iter unifyIterator) error {
 		return err
 	}
 
-	if ir.Empty() {
+	// default functions aren't supported:
+	// https://github.com/open-policy-agent/opa/issues/2445
+	if len(ir.Rules) == 0 {
 		return nil
 	}