Fix NPE on empty registry in configuration

Author: vsbogd

sdk/src/main/java/io/singularitynet/sdk/client/JsonConfiguration.java

@@ -60,7 +60,7 @@ public byte[] getSignerPrivateKey() {
 
     @Override
     public Address getRegistryAddress() {
-        return new Address(registryAddress);
+        return registryAddress == null ? null : new Address(registryAddress);
     }
 
     @Override

sdk/src/test/java/io/singularitynet/sdk/client/JsonConfigurationTest.java

@@ -62,6 +62,15 @@ public void configureRegistryAddress() {
         assertEquals("Registry address", new Address(registryAddress), config.getRegistryAddress());
     }
 
+    @Test
+    public void configureNoRegistryAddress() {
+        String json = "{}";
+
+        Configuration config = new JsonConfiguration(json);
+
+        assertNull("Registry address", config.getRegistryAddress());
+    }
+
     @Test
     public void configureMultiPartyEscrowAddress() {
         String mpeAddress = "0x8FB1dC8df86b388C7e00689d1eCb533A160B4D0C";