Deserialize null capabilities in Device entities as empty sets

service/src/test/java/org/whispersystems/textsecuregcm/storage/DeviceTest.java

@@ -6,11 +6,15 @@
 package org.whispersystems.textsecuregcm.storage;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 
 import java.time.Duration;
 import java.time.Instant;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
+import org.whispersystems.textsecuregcm.util.SystemMapper;
 
 class DeviceTest {
 
@@ -42,4 +46,25 @@ class DeviceTest {
     assertEquals(expectExpired, device.isExpired());
   }
 
+  @Test
+  void deserializeCapabilities() throws JsonProcessingException {
+    {
+      final Device device = SystemMapper.jsonMapper().readValue("""
+          {
+            "capabilities": null
+          }
+          """, Device.class);
+
+      assertNotNull(device.getCapabilities(),
+          "Device deserialization should populate null capabilities with an empty set");
+    }
+
+    {
+      final Device device = SystemMapper.jsonMapper().readValue("{}", Device.class);
+
+      assertNotNull(device.getCapabilities(),
+          "Device deserialization should populate null capabilities with an empty set");
+    }
+  }
+
 }