Shared/Java: Make the 'isNull' interface slightly prettier.

shared/typeflow/codeql/typeflow/TypeFlow.qll

@@ -42,6 +42,12 @@ signature module TypeFlowInput<LocationSig Location> {
   /** Holds if `n` represents a `null` value. */
   predicate isNullValue(TypeFlowNode n);
 
+  /**
+   * Holds if `n` should be excluded from the set of null values even if
+   * the null analysis determines that `n` is always null.
+   */
+  default predicate isExcludedFromNullAnalysis(TypeFlowNode n) { none() }
+
   /** A type. */
   class Type {
     /** Gets a textual representation of this type. */

shared/typeflow/codeql/typeflow/internal/TypeFlowImpl.qll

@@ -10,6 +10,9 @@ module TypeFlow<LocationSig Location, TypeFlowInput<Location> I> {
     isNullValue(n)
     or
     exists(TypeFlowNode mid | isNull(mid) and step(mid, n))
+    or
+    forex(TypeFlowNode mid | I::joinStep(mid, n) | isNull(mid)) and
+    not isExcludedFromNullAnalysis(n)
   }
 
   /**