add unit test to prevent NPE when user agent string is null

src/main/java/org/signal/storageservice/util/ua/UserAgentUtil.java

@@ -23,6 +23,9 @@ public class UserAgentUtil {
     }
 
     public static ClientPlatform getPlatformFromUserAgentString(final String userAgentString) throws UnrecognizedUserAgentException {
+      if (userAgentString == null) {
+        throw new UnrecognizedUserAgentException();
+      }
       final Matcher standardUaMatcher = STANDARD_UA_PATTERN.matcher(userAgentString);
 
       if (standardUaMatcher.matches()) {

src/test/java/org/signal/storageservice/util/ua/UserAgentUtilTest.java

@@ -0,0 +1,21 @@
+package org.signal.storageservice.util.ua;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.junit.jupiter.api.Test;
+
+class UserAgentUtilTest {
+  @Test
+  void testGetPlatformFromUserAgentString() throws UnrecognizedUserAgentException {
+    String userAgentString = "Signal-Android/12.35.42 Android/31";
+    ClientPlatform platform = UserAgentUtil.getPlatformFromUserAgentString(userAgentString);
+    assertThat(platform).isSameAs(ClientPlatform.ANDROID);
+  }
+
+  @Test
+  void testGetPlatformFromUserAgentString_null() {
+    String userAgentString = null;
+    assertThatThrownBy(() -> UserAgentUtil.getPlatformFromUserAgentString(userAgentString)).isInstanceOf(UnrecognizedUserAgentException.class);
+  }
+}