Fix method template signature broken in #639 (#700)

Author: chingor13

google-http-client/src/main/java/com/google/api/client/util/Data.java

@@ -108,7 +108,7 @@ public class Data {
    * @return magic object instance that represents the "null" value (not Java {@code null})
    * @throws IllegalArgumentException if unable to create a new instance
    */
-  public static <T> T nullOf(Class<?> objClass) {
+  public static <T> T nullOf(Class<T> objClass) {
     // ConcurrentMap.computeIfAbsent is explicitly NOT used in the following logic. The
     // ConcurrentHashMap implementation of that method BLOCKS if the mappingFunction triggers
     // modification of the map which createNullInstance can do depending on the state of class

google-http-client/src/test/java/com/google/api/client/util/DataTest.java

@@ -14,6 +14,7 @@
 
 package com.google.api.client.util;
 
+import com.google.common.collect.ImmutableMap;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.lang.reflect.TypeVariable;
@@ -68,6 +69,12 @@ public void testNullOf() {
     }
   }
 
+  public void testNullOfTemplateTypes() {
+    String nullValue = Data.nullOf(String.class);
+    Map<String, String> nullField = ImmutableMap.of("v", nullValue);
+    assertEquals(nullValue, nullField.get("v"));
+  }
+
   public void testIsNull() {
     assertTrue(Data.isNull(Data.NULL_BOOLEAN));
     assertTrue(Data.isNull(Data.NULL_STRING));