Java/Shared: Share more 'isNull' computations.

java/ql/lib/semmle/code/java/dataflow/TypeFlow.qll

@@ -125,7 +125,7 @@ private module Input implements TypeFlowInput<J::Location> {
   /**
    * Holds if `null` is the only value that flows to `n`.
    */
-  predicate isNull(TypeFlowNode n) {
+  predicate isNullValue(TypeFlowNode n) {
     n.asExpr() instanceof NullLiteral
     or
     exists(LocalVariableDeclExpr decl |
@@ -134,9 +134,7 @@ private module Input implements TypeFlowInput<J::Location> {
       not exists(decl.getInit())
     )
     or
-    exists(TypeFlowNode mid | isNull(mid) and step(mid, n))
-    or
-    forex(TypeFlowNode mid | joinStep0(mid, n) | isNull(mid)) and
+    forex(TypeFlowNode mid | joinStep0(mid, n) | Make<J::Location, Input>::isNull(mid)) and
     // Fields that are never assigned a non-null value are probably set by
     // reflection and are thus not always null.
     not exists(n.asField())

shared/typeflow/codeql/typeflow/TypeFlow.qll

@@ -39,10 +39,8 @@ signature module TypeFlowInput<LocationSig Location> {
    */
   predicate step(TypeFlowNode n1, TypeFlowNode n2);
 
-  /**
-   * Holds if `null` is the only value that flows to `n`.
-   */
-  predicate isNull(TypeFlowNode n);
+  /** Holds if `n` represents a `null` value. */
+  predicate isNullValue(TypeFlowNode n);
 
   /** A type. */
   class Type {

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

@@ -5,6 +5,13 @@ private import codeql.util.Unit
 module TypeFlow<LocationSig Location, TypeFlowInput<Location> I> {
   private import I
 
+  /** Holds if `null` is the only value that flows to `n`. */
+  predicate isNull(TypeFlowNode n) {
+    isNullValue(n)
+    or
+    exists(TypeFlowNode mid | isNull(mid) and step(mid, n))
+  }
+
   /**
    * Holds if data can flow from `n1` to `n2` in one step, `n1` is not necessarily
    * functionally determined by `n2`, and `n1` might take a non-null value.