getTangle() returns null if tangleID is unknown

lib/index.js

@@ -1145,10 +1145,15 @@ function initDB(peer, config) {
 
   /**
    * @param {MsgID} tangleID
-   * @returns {DBTangle}
+   * @returns {DBTangle | null}
    */
   function getTangle(tangleID) {
-    return new DBTangle(tangleID, records(), get)
+    const tangle = new DBTangle(tangleID, records(), get)
+    if (tangle.size > 0) {
+      return tangle
+    } else {
+      return null
+    }
   }
 
   function* msgs() {

test/getTangle.test.js

@@ -106,6 +106,13 @@ test('getTangle()', async (t) => {
     tangle = peer.db.getTangle(rootPost)
   }
 
+  await t.test('getTangle unknown ID returns null', (t) => {
+    assert.equal(
+      peer.db.getTangle('Lq6xwbdvGVmSsY3oYRugpZ3DY8chX9SLhRhjJKyZHQn'),
+      null
+    )
+  })
+
   await t.test('Tangle.has', (t) => {
     assert.equal(tangle.has(rootPost), true, 'has rootPost')
     assert.equal(tangle.has(reply1Lo), true, 'has reply1Lo')