HasMetadata item function cannot distinguish between "not present" and "set to empty"

I had the need to filter components based on the presence of metadata, regardless of its value.

The HasMetadata item function, however, checks for the metadata _value_, rather than just the presence of the metadata name, so items with the metadata, but an empty value, are improperly filtered out.

The following snippet showcases the issue:

``` XML








Bar

```

This renders "Bar", but not "Foo", which does contain the metadata item.

I know this would be a breaking change, so maybe a new item function named `HasMetadataName` could be provided for this behavior?