Fix net.i2p.util.Addresses::getIP empty string

The standard library behavior is to return the localhost when null or an empty string is passed.
getIP seeked to override that behavior, but didn't treat the empty string case.

core/java/src/net/i2p/util/Addresses.java

@@ -399,7 +399,7 @@ public abstract class Addresses {
      *  @since 0.9.3
      */
     public static byte[] getIP(String host) {
-        if (host == null)
+        if (host == null || host.isEmpty())
             return null;
         byte[] rv;
         synchronized (_IPAddress) {

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

@@ -19,10 +19,7 @@ public class AddressesTest {
 
     @Test
     public void getIPEmptyString() {
-        byte[] address = {
-            127, 0, 0, 1
-        };
-        assertArrayEquals(address, Addresses.getIP(""));
+        assertNull(Addresses.getIP(""));
     }
 
     @Test