test: Add ConvertToHashTest fall through case

Nothing can be resolved at all.

core/java/test/junit/net/i2p/util/ConvertToHashTest.java

@@ -58,4 +58,12 @@ public class ConvertToHashTest {
         assertNotNull(hash);
         assertEquals(hash.toBase32(), zzzB32I2P);
     }
+
+    /**
+     * The case where a destination cannot be resolved at all
+     */
+    @Test
+    public void getHashResolveDestinationFail() {
+        assertNull(ConvertToHash.getHash("unknown.i2p"));
+    }
 }