[ANCHOR-346] Fix trailing comma in assets JSON file causes a "null" a…

…ssets (#1369)

### Description

- Fix trailing comma in assets JSON file causes a "null" assets

Although the trailing comma in JSON does not cause gson parsing failure,
it creates a `null` entry in the result `Assets` object. The PR removes
the `null` entry.

### Context

- Bug fixe.

### Testing
- `./gradlew test`

### Documentation
N/A
### Known limitations

N/A

core/src/main/java/org/stellar/anchor/asset/DefaultAssetService.java

@@ -66,6 +66,7 @@ public static DefaultAssetService fromYaml(String assetsYaml) throws InvalidConf
   public static DefaultAssetService fromJson(String assetsJson) throws InvalidConfigException {
     DefaultAssetService assetService = new DefaultAssetService();
     assetService.assets = gson.fromJson(assetsJson, Assets.class);
+    assetService.assets.assets.removeIf(Objects::isNull);
     AssetServiceValidator.validate(assetService);
     return assetService;
   }
@@ -88,7 +89,6 @@ public List<AssetInfo> listAllAssets() {
   @Override
   public AssetInfo getAsset(String code) {
     for (AssetInfo asset : assets.getAssets()) {
-      // FIXME: ANCHOR-346
       if (asset != null && asset.getCode().equals(code)) {
         return asset;
       }

core/src/test/kotlin/org/stellar/anchor/asset/DefaultAssetServiceTest.kt

@@ -19,6 +19,7 @@ internal class DefaultAssetServiceTest {
   private val gson: Gson = GsonUtils.getInstance()
 
   @BeforeEach fun setup() {}
+
   @ParameterizedTest
   @ValueSource(strings = ["test_assets.json", "test_assets.yaml"])
   fun `test assets loading and listing`(filename: String) {
@@ -88,6 +89,12 @@ internal class DefaultAssetServiceTest {
     }
   }
 
+  @Test
+  fun `test trailing comma in JSON does not result in null element`() {
+    val assetsService = DefaultAssetService.fromJson(trailingCommaInAssets)
+    assert(assetsService.assets.assets.all { it != null })
+  }
+
   // This is supposed to match the result from loading test_assets.json file.
   private val expectedAssetsJson =
     """
@@ -387,3 +394,17 @@ internal class DefaultAssetServiceTest {
     """
       .trimIndent()
 }
+
+val trailingCommaInAssets =
+  """
+  {
+    "assets": [
+      {
+        "schema": "stellar",
+        "code": "native",
+        "significant_decimals": 7
+      },
+    ]
+  }
+"""
+    .trimIndent()