Mark Attribute#getValue as @Nullable

Change-Id: I172c43ff2e3e682f38a91ce288190245fa5d5ebe

org.eclipse.jgit/src/org/eclipse/jgit/attributes/Attribute.java

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/attributes/Attribute.java b/org.eclipse.jgit/src/org/eclipse/jgit/attributes/Attribute.java
index fe3e22a..9c4d870 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/attributes/Attribute.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/attributes/Attribute.java
@@ -9,6 +9,8 @@
  */
 package org.eclipse.jgit.attributes;
 
+import org.eclipse.jgit.annotations.Nullable;
+
 /**
  * Represents an attribute.
  * <p>
@@ -139,6 +141,7 @@ public State getState() {
 	 *
 	 * @return the attribute value (may be <code>null</code>)
 	 */
+	@Nullable
 	public String getValue() {
 		return value;
 	}