Correctly mark checksums as optional

This makes the checksums available for packages and installers
optional. It updates the parser to correctly return an optional string,
rather than defaulting to an always-present empty string.

Fix: #5

Author: io7m

net.adoptopenjdk.v3.api/src/main/java/net/adoptopenjdk/v3/api/AOV3InstallerType.java

@@ -33,7 +33,7 @@ public interface AOV3InstallerType
    * @return The checksum of the data
    */
 
-  String checksum();
+  Optional<String> checksum();
 
   /**
    * @return A link to the checksum of the data

net.adoptopenjdk.v3.tests/src/test/java/net/adoptopenjdk/v3/tests/AOV3ResponseParsersTest.java

@@ -16,6 +16,7 @@
 
 import net.adoptopenjdk.v3.api.AOV3Error;
 import net.adoptopenjdk.v3.api.AOV3ExceptionParseFailed;
+import net.adoptopenjdk.v3.api.AOV3Installer;
 import net.adoptopenjdk.v3.vanilla.AOV3ResponseParsers;
 import net.adoptopenjdk.v3.vanilla.AOV3ResponseParsersType;
 import org.apache.commons.io.input.BrokenInputStream;
@@ -274,6 +275,37 @@ public void testAssetsForLatest()
     Assertions.assertEquals(0, this.errors.size());
   }
 
+  @Test
+  public void testAssetsForReleases8()
+    throws Exception
+  {
+    final var stream = resource("releases8.json");
+    final var parser =
+      this.parsers.createParser(this::logError, URI.create("urn:test"), stream);
+    final var releases = parser.parseAssetsForRelease();
+
+    for (final var release : releases) {
+      LOG.debug("release {}", release.id());
+      for (final var binary : release.binaries()) {
+        LOG.debug("release {} binary {}", release.id(), binary.package_().name());
+        final var packChecksumOpt = binary.package_().checksum();
+        if (packChecksumOpt.isPresent()) {
+          final var checksum = packChecksumOpt.get();
+          Assertions.assertFalse(checksum.isBlank());
+        }
+
+        final var instChecksumOpt =
+          binary.installer().flatMap(AOV3Installer::checksum);
+        if (instChecksumOpt.isPresent()) {
+          final var checksum = instChecksumOpt.get();
+          Assertions.assertFalse(checksum.isBlank());
+        }
+      }
+    }
+
+    Assertions.assertEquals(0, this.errors.size());
+  }
+
   private void testAssetsForReleaseFuzzOnce(
     final Integer index)
     throws Exception