Method
http://download.java.net/jdk8/docs/api/java/util/Map.html#computeIfPresent%28K,%20java.util.function.BiFunction%29

specifies:

Throws:
    NullPointerException - if ... the remappingFunction is null


NPE is not thrown if the given key is not present, consider the following code sample:

---
        new LinkedHashMap().computeIfPresent("a", (BiFunction) null);
        new TreeMap().computeIfPresent("a", (BiFunction) null);
        new HashMap().computeIfPresent("a", (BiFunction) null);

        new HashMap(){{put("a", "a");}}.computeIfPresent("a", (BiFunction) null);
---

Only the last line will throw NPE.

The following tests will fail:
api/java_util/TreeMap/Map8Tests.html#Map8Tests[computeIfPresent_mappingFunctionIsNull]
api/java_util/Collections/synchronizd/SynchronizedSortedMap8Tests.html#SynchronizedSortedMap8Tests[computeIfPresent_mappingFunctionIsNull]
api/java_util/Collections/synchronizd/SynchronizedMap8Tests.html#SynchronizedMap8Tests[computeIfPresent_mappingFunctionIsNull]
api/java_util/IdentityHashMap/Map8Tests.html#Map8Tests[computeIfPresent_mappingFunctionIsNull]
api/java_util/HashMap/Map8Tests.html#Map8Tests[computeIfPresent_mappingFunctionIsNull]
api/java_util/LinkedHashMap/Map8Tests.html#Map8Tests[computeIfPresent_mappingFunctionIsNull]
api/java_util/WeakHashMap/Map8Tests.html#Map8Tests[computeIfPresent_mappingFunctionIsNull]