Report all errors as [nullness]. (#194)

Co-authored-by: Werner Dietl <[email protected]>

Author: cpovirk

src/main/java/com/google/jspecify/nullness/NullSpecChecker.java

@@ -69,6 +69,11 @@ public NavigableSet<String> getSuppressWarningsPrefixes() {
     return prefixes;
   }
 
+  @Override
+  protected String suppressWarningsString(String messageKey) {
+    return "nullness";
+  }
+
   @Override
   public void initChecker() {
     super.initChecker();