test: Add test case for empty external Id

Author: enmustafaj

src/main/java/org/zendesk/client/v2/Zendesk.java

@@ -1423,8 +1423,8 @@ public JobStatus deleteOrganizations(long... ids) {
     }
 
     public Iterable<Organization> lookupOrganizationsByExternalId(String externalId) {
-        if (externalId == null) {
-            throw new IllegalArgumentException("External ID must not be null");
+        if (externalId == null || externalId.length() == 0) {
+            throw new IllegalArgumentException("External ID must not be null or length 0");
         }
         return new PagedIterable<>(
                 tmpl("/organizations/search.json{?external_id}").set("external_id", externalId),

src/test/java/org/zendesk/client/v2/RealSmokeTest.java

@@ -97,6 +97,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assume.assumeThat;
@@ -1583,6 +1584,21 @@ public void createOrganizationMemberships() throws Exception {
         }
     }
 
+    @Test
+    public void lookupOrganizationByExternalId() throws Exception {
+        createClientWithTokenOrPassword();
+
+        Organization newOrganization = newTestOrganization();
+        newOrganization.setExternalId("i");
+        Organization resultOrganization = instance.createOrganization(newOrganization);
+        assertNotNull(resultOrganization);
+
+        Iterable<Organization> or =  instance.lookupOrganizationsByExternalId("i");
+        assertTrue(or.iterator().hasNext());
+
+        assertThrows(IllegalArgumentException.class, () -> instance.lookupOrganizationsByExternalId(""));
+    }
+
     @Test
     public void getGroups() throws Exception {
         createClientWithTokenOrPassword();