#448 Specify @NotNull on OptionalPropertyType#convert

Author: ljacqu

src/main/java/ch/jalu/configme/properties/types/OptionalPropertyType.java

@@ -25,7 +25,7 @@ public OptionalPropertyType(PropertyType<T> valueType) {
     }
 
     @Override
-    public @Nullable Optional<T> convert(@Nullable Object object, @NotNull ConvertErrorRecorder errorRecorder) {
+    public @NotNull Optional<T> convert(@Nullable Object object, @NotNull ConvertErrorRecorder errorRecorder) {
         if (object != null) {
             return Optional.ofNullable(valueType.convert(object, errorRecorder));
         }