Revert "NULL_LITERAL expressions may always be null (uber#749)"

This reverts commit e4dfeac.

nullaway/src/main/java/com/uber/nullaway/NullAway.java

@@ -2197,14 +2197,14 @@ private boolean mayBeNullExpr(VisitorState state, ExpressionTree expr) {
       // obviously not null
       return false;
     }
-    if (expr.getKind() == Tree.Kind.NULL_LITERAL) {
-      // obviously null, so same as for above literals we return early without consulting handlers
-      return true;
-    }
     // the logic here is to avoid doing dataflow analysis whenever possible
     Symbol exprSymbol = ASTHelpers.getSymbol(expr);
     boolean exprMayBeNull;
     switch (expr.getKind()) {
+      case NULL_LITERAL:
+        // obviously null
+        exprMayBeNull = true;
+        break;
       case ARRAY_ACCESS:
         // unsound!  we cannot check for nullness of array contents yet
         exprMayBeNull = false;