FINERACT-1724: Client search API fix when external ID is null

fineract-provider/src/main/java/org/apache/fineract/infrastructure/core/jersey/converter/ExternalIdJsonConverter.java

@@ -42,6 +42,8 @@ public class ExternalIdJsonConverter implements JsonConverter<ExternalId> {
     public void convertToJson(ExternalId value, JsonGenerator generator) throws IOException {
         if (value != null && !value.isEmpty()) {
             generator.writeString(value.getValue());
+        } else {
+            generator.writeNull();
         }
     }
 

integration-tests/src/test/java/org/apache/fineract/integrationtests/client/ClientSearchTest.java

@@ -73,6 +73,32 @@ public class ClientSearchTest {
         assertThat(result.getTotalPages()).isEqualTo(3);
     }
 
+    @Test
+    public void testClientSearchWorks_WhenNoExternalIdForClients() {
+        // given
+        String lastname = Utils.randomStringGenerator("Client_LastName_", 5);
+        PostClientsRequest request1 = ClientHelper.defaultClientCreationRequest();
+        request1.setExternalId(null);
+        request1.setLastname(lastname);
+        clientHelper.createClient(request1);
+
+        PostClientsRequest request2 = ClientHelper.defaultClientCreationRequest();
+        request2.setExternalId(null);
+        request2.setLastname(lastname);
+        clientHelper.createClient(request2);
+
+        PostClientsRequest request3 = ClientHelper.defaultClientCreationRequest();
+        request3.setExternalId(null);
+        request3.setLastname(lastname);
+        clientHelper.createClient(request3);
+        // when
+        PageClientSearchData result = clientHelper.searchClients(lastname, 0, 1);
+        // then
+        assertThat(result.getTotalElements()).isEqualTo(3);
+        assertThat(result.getNumberOfElements()).isEqualTo(1);
+        assertThat(result.getTotalPages()).isEqualTo(3);
+    }
+
     @Test
     public void testClientSearchWorks_WithLastnameTextOnDefaultOrdering() {
         // given