TreeMap in JDK15 deviates from spec

[Phil Smith]

Hello, I submitted bug report 9068554 recently, but I consider this a
pretty big issue and I'd like for this not to get lost in triage.

With JDK-8176894, a specialized implementation for computeIfAbsent was
added to TreeMap, however it doesn't handle null values correctly.
Spec states a mapping to null is equivalent to no mapping being
present, however code will `return t.value;` without checking that
value. The interface default, for reference, looks like `if ((v =
get(key)) == null)`.

A simple repro follows

TreeMap treemap = new TreeMap();
treemap.put("a", null);
System.out.println(treemap.computeIfAbsent("a", key -> "default"));
System.out.println(treemap.get("a"));

HashMap hashmap = new HashMap();
hashmap.put("a", null);
System.out.println(hashmap.computeIfAbsent("a", key -> "default"));
System.out.println(hashmap.get("a"));

Phil