Remove incorrect null/ entry from .gitignore

The null/ directory was accidentally created and incorrectly added to
.gitignore. This removes the incorrect gitignore entry.

Addresses review: apache#394 (review)

Author: cowwoc

.gitignore

@@ -15,4 +15,3 @@ out/
 .java-version
 .checkstyle
 .factorypath
-null/