Fix NPE on configuring default MPE contract address

Author: vsbogd

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

@@ -65,7 +65,7 @@ public Address getRegistryAddress() {
 
     @Override
     public Address getMultiPartyEscrowAddress() {
-        return new Address(multiPartyEscrowAddress);
+        return multiPartyEscrowAddress == null ? null : new Address(multiPartyEscrowAddress);
     }
 
 }

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

@@ -63,7 +63,7 @@ public void configureRegistryAddress() {
     }
 
     @Test
-    public void configureNoRegistryAddress() {
+    public void configureDefaultRegistryAddress() {
         String json = "{}";
 
         Configuration config = new JsonConfiguration(json);
@@ -83,4 +83,13 @@ public void configureMultiPartyEscrowAddress() {
         assertEquals("MultiPartyEscrow address", new Address(mpeAddress), config.getMultiPartyEscrowAddress());
     }
 
+    @Test
+    public void configureDefaultMultiPartyEscrowAddress() {
+        String json = "{}";
+
+        Configuration config = new JsonConfiguration(json);
+
+        assertNull("MultiPartyEscrow address", config.getMultiPartyEscrowAddress());
+    }
+
 }