commons: add null check to nanToZero method

Motivation:

https://rb.dcache.org/r/13867/
neglected the possibility that the value could be null.

In the context of the histogram code, this can occur,
particularly at initialization.

Modification:

Check for null.

Result:

No exception.

Target: master
Request: 8.2  (update pull request)
Request: 8.1  (update pull request)
Request: 8.0  (update pull request)
Request: 7.2  (update pull request)
Pull-request:  #6980
Pull-request:  #6981
Pull-request:  #6982
Pull-request:  #6983
Requires-notes: no
Patch: https://rb.dcache.org/r/13868/
Acked-by: Paul

modules/common/src/main/java/org/dcache/util/MathUtils.java

@@ -78,10 +78,10 @@ public static long subWithInfinity(long a, long b) {
      *  convert NaN to 0.0.
      *
      * @param value the double result of the computation.
-     * @return 0.0 if the value is NaN; otherwise, the value.
+     * @return 0.0 if the value is NaN; otherwise, the value (including null).
      */
     public static Double nanToZero(Double value) {
-        return Double.isNaN(value) ? 0.0 : value;
+        return value != null && Double.isNaN(value) ? 0.0 : value;
     }
 
 }