Fix null array values in HOCON/JSON config parser. (#2731)

Resolves #2720 (follow-up)

config/hocon/src/main/java/io/helidon/config/hocon/HoconConfigParser.java

@@ -168,7 +168,12 @@ private static ListNode fromList(ConfigList list) {
             } else if (value instanceof ConfigObject) {
                 builder.addObject(fromConfig((ConfigObject) value));
             } else {
-                builder.addValue(value.unwrapped().toString());
+                Object unwrapped = value.unwrapped();
+                if (unwrapped == null) {
+                    builder.addValue("");
+                } else {
+                    builder.addValue(String.valueOf(unwrapped));
+                }
             }
         });
         return builder.build();

config/hocon/src/test/java/io/helidon/config/hocon/HoconConfigParserTest.java

@@ -48,6 +48,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.hasItems;
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
@@ -292,6 +293,9 @@ void testParserFromJson() {
 
         property = config.get("nulls.null").asString().asOptional();
         assertThat(property, is(Optional.of("")));
+
+        List<String> properties = config.get("nulls-array").asList(String.class).get();
+        assertThat(properties, hasItems("test", ""));
     }
 
     //

config/hocon/src/test/resources/config.json

@@ -9,5 +9,6 @@
     },
     "nulls": {
         "null": null
-    }
+    },
+    "nulls-array": ["test", null]
 }