wit: do not emit WIT for empty packages

testdata/issues/issue347.wit

@@ -0,0 +1,16 @@
+/// This comment must exist to trigger failure.
+package issues:issue347;
+
+world foo-command {
+  import issues:inner/custom;
+}
+
+package issues:inner {
+	interface custom {
+		words: func(text: string) -> list<string>;
+	}
+
+	world the-world {
+		export custom;
+	}
+}

testdata/issues/issue347.wit.json

@@ -0,0 +1,78 @@
+{
+  "worlds": [
+    {
+      "name": "the-world",
+      "imports": {},
+      "exports": {
+        "interface-0": {
+          "interface": {
+            "id": 0
+          }
+        }
+      },
+      "package": 0
+    },
+    {
+      "name": "foo-command",
+      "imports": {
+        "interface-0": {
+          "interface": {
+            "id": 0
+          }
+        }
+      },
+      "exports": {},
+      "package": 1
+    }
+  ],
+  "interfaces": [
+    {
+      "name": "custom",
+      "types": {},
+      "functions": {
+        "words": {
+          "name": "words",
+          "kind": "freestanding",
+          "params": [
+            {
+              "name": "text",
+              "type": "string"
+            }
+          ],
+          "result": 0
+        }
+      },
+      "package": 0
+    }
+  ],
+  "types": [
+    {
+      "name": null,
+      "kind": {
+        "list": "string"
+      },
+      "owner": null
+    }
+  ],
+  "packages": [
+    {
+      "name": "issues:inner",
+      "interfaces": {
+        "custom": 0
+      },
+      "worlds": {
+        "the-world": 0
+      }
+    },
+    {
+      "name": "issues:issue347",
+      "docs": {
+        "contents": "This comment must exist to trigger failure."
+      },
+      "interfaces": {},
+      "worlds": {
+        "foo-command": 1
+      }
+    }
+  ]
+}

testdata/issues/issue347.wit.json.golden.wit

@@ -0,0 +1,16 @@
+/// This comment must exist to trigger failure.
+package issues:issue347;
+
+world foo-command {
+	import issues:inner/custom;
+}
+
+package issues:inner {
+	interface custom {
+		words: func(text: string) -> list<string>;
+	}
+
+	world the-world {
+		export custom;
+	}
+}

wit/wit.go

@@ -1206,9 +1206,9 @@ func (p *Package) WIT(ctx Node, name string) string {
 		b.WriteString("}\n")
 	}
 	// Return empty string if package has no contents
-	// if i == 0 {
-	// 	return ""
-	// }
+	if filter != nil && i == 0 {
+		return ""
+	}
 	return b.String()
 }