Add unit tests

Author: Athira M

src/test/java/com/google/firebase/remoteconfig/ServerTemplateImplTest.java

@@ -63,6 +63,21 @@ public void testServerTemplateWithoutCacheValueThrowsException()
     assertEquals("JSON String must not be null or empty.", error.getMessage());
   }
 
+  @Test
+  public void testEvaluateWithoutContextReturnsDefaultValue()
+      throws FirebaseRemoteConfigException {
+    KeysAndValues defaultConfig = new KeysAndValues.Builder().build();
+    ServerTemplate template =
+        new ServerTemplateImpl.Builder(null)
+            .defaultConfig(defaultConfig)
+            .cachedTemplate(cacheTemplate)
+            .build();
+
+    ServerConfig evaluatedConfig = template.evaluate();
+
+    assertEquals("Default value", evaluatedConfig.getString("Custom"));
+  }
+
   @Test
   public void testEvaluateCustomSignalReturnsDefaultValue() throws FirebaseRemoteConfigException {
     KeysAndValues defaultConfig = new KeysAndValues.Builder().build();