Ensure nullness correctness using Checker Framework.

Change-Id: Ie5a7749194313664a206e44597091a62afca9bdb
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java
index 3a8185c..cfe3635 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/ByteString.java
@@ -10,6 +10,7 @@
 import java.util.HexFormat;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * A wrapper for a {@code byte[]} that implements {@link #equals(Object)}, {@link #hashCode()}, and
@@ -31,7 +32,7 @@
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     return (o instanceof ByteString byteString) && Arrays.equals(bytes, byteString.bytes);
   }