empty gsa.hostname is invalid

this is an additional check to it being necessary

src/com/google/enterprise/adaptor/Config.java

diff --git a/src/com/google/enterprise/adaptor/Config.java b/src/com/google/enterprise/adaptor/Config.java
index dd187ea..56db897 100644
--- a/src/com/google/enterprise/adaptor/Config.java
+++ b/src/com/google/enterprise/adaptor/Config.java
@@ -720,6 +720,9 @@
 
   public void validate() {
     validate(config);
+    if ("".equals(getGsaHostname().trim())) {
+      throw new InvalidConfigurationException("gsa.hostname cannot be empty");
+    }
   }
 
   private void validate(Properties config) {

test/com/google/enterprise/adaptor/ConfigTest.java

diff --git a/test/com/google/enterprise/adaptor/ConfigTest.java b/test/com/google/enterprise/adaptor/ConfigTest.java
index 639d0f4..df4ca4c 100644
--- a/test/com/google/enterprise/adaptor/ConfigTest.java
+++ b/test/com/google/enterprise/adaptor/ConfigTest.java
@@ -49,6 +49,22 @@
   }
 
   @Test
+  public void testEmptyGsaHostname() {
+    // Requires gsa.hostname to be non-empty
+    config.setValue("gsa.hostname", "");
+    thrown.expect(InvalidConfigurationException.class);
+    config.validate();
+  }
+
+  @Test
+  public void testWhitespaceOnlyGsaHostname() {
+    // Requires gsa.hostname to be non-empty
+    config.setValue("gsa.hostname", "  ");
+    thrown.expect(InvalidConfigurationException.class);
+    config.validate();
+  }
+
+  @Test
   public void testAddDuplicateKeyWithValue() {
     config.addKey("somekey", "value");
     thrown.expect(IllegalStateException.class);